add feature gate for managed boxes
add feature gate for managed boxes

I'll flip this on after doing a snapshot. This syntax may or may not
stay around, and managed boxes are currently not very useful. They have
the same overall performance characteristics as `std::rc::Rc`, but are
significantly slower, allocate larger boxes and hold onto the memory
beyond when it is needed due to lacking move semantics.

There are currently two useful aspects of the type:

* the dereference sugar, which we should implement for `Rc`
* the annihilator freeing cycles at the end of the task

kwantam/rust,michaelballantyne/rust-gpu,TheNeikos/rust,fabricedesre/rust,SiegeLord/rust,reem/rust,kwantam/rust,barosl/rust,aneeshusa/rust,stepancheg/rust-ide-rust,GBGamer/rust,rohitjoshi/rust,AerialX/rust,nwin/rust,krzysz00/rust,mitsuhiko/rust,philyoon/rust,seanrivera/rust,seanrivera/rust,ejjeong/rust,mitsuhiko/rust,pythonesque/rust,XMPPwocky/rust,KokaKiwi/rust,0x73/rust,robertg/rust,0x73/rust,servo/rust,aturon/rust,jashank/rust,pythonesque/rust,robertg/rust,bluss/rand,quornian/rust,ejjeong/rust,aidancully/rust,kimroen/rust,pczarn/rust,pythonesque/rust,emk/rust,servo/rust,pshc/rust,AerialX/rust,untitaker/rust,nham/rust,hauleth/rust,michaelballantyne/rust-gpu,servo/rust,defuz/rust,pelmers/rust,rohitjoshi/rust,dwillmer/rust,jashank/rust,jbclements/rust,TheNeikos/rust,miniupnp/rust,andars/rust,ebfull/rust,pczarn/rust,zaeleus/rust,cllns/rust,ktossell/rust,GBGamer/rust,rprichard/rust,mvdnes/rust,bombless/rust,richo/rust,ktossell/rust,bombless/rust,mitsuhiko/rust,stepancheg/rust-ide-rust,Ryman/rust,omasanori/rust,zachwick/rust,dinfuehr/rust,pczarn/rust,aturon/rust,nham/rust,AerialX/rust-rt-minimal,SiegeLord/rust,rprichard/rust,zachwick/rust,l0kod/rust,AerialX/rust,bombless/rust,robertg/rust,reem/rust,victorvde/rust,XMPPwocky/rust,vhbit/rust,jroesch/rust,reem/rust,arthurprs/rand,mitsuhiko/rust,pczarn/rust,rohitjoshi/rust,richo/rust,Ryman/rust,vhbit/rust,mihneadb/rust,pshc/rust,shepmaster/rand,kwantam/rust,miniupnp/rust,XMPPwocky/rust,jroesch/rust,aidancully/rust,nwin/rust,untitaker/rust,jroesch/rust,nwin/rust,GBGamer/rust,michaelballantyne/rust-gpu,l0kod/rust,rprichard/rust,philyoon/rust,richo/rust,vhbit/rust,jashank/rust,aepsil0n/rust,untitaker/rust,aneeshusa/rust,TheNeikos/rust,mahkoh/rust,zachwick/rust,dwillmer/rust,sae-bom/rust,miniupnp/rust,vhbit/rust,hauleth/rust,P1start/rust,mdinger/rust,cllns/rust,ebfull/rand,l0kod/rust,sae-bom/rust,reem/rust,P1start/rust,aneeshusa/rust,omasanori/rust,gifnksm/rust,jbclements/rust,jbclements/rust,seanrivera/rust,mvdnes/rust,reem/rust,sae-bom/rust,aepsil0n/rust,victorvde/rust,zubron/rust,sae-bom/rust,krzysz00/rust,carols10cents/rust,ejjeong/rust,krzysz00/rust,ejjeong/rust,gifnksm/rust,jroesch/rust,mahkoh/rust,aepsil0n/rust,dinfuehr/rust,aneeshusa/rust,AerialX/rust-rt-minimal,SiegeLord/rust,jroesch/rust,krzysz00/rust,l0kod/rust,stepancheg/rust-ide-rust,jbclements/rust,ebfull/rust,jashank/rust,mihneadb/rust,krzysz00/rust,P1start/rust,AerialX/rust-rt-minimal,dwillmer/rust,andars/rust,pshc/rust,P1start/rust,mvdnes/rust,XMPPwocky/rust,mdinger/rust,graydon/rust,gifnksm/rust,pythonesque/rust,TheNeikos/rust,barosl/rust,nwin/rust,pshc/rust,mvdnes/rust,emk/rust,avdi/rust,philyoon/rust,vhbit/rust,pythonesque/rust,servo/rust,zachwick/rust,michaelballantyne/rust-gpu,andars/rust,XMPPwocky/rust,sae-bom/rust,graydon/rust,graydon/rust,hauleth/rust,zaeleus/rust,aepsil0n/rust,barosl/rust,carols10cents/rust,SiegeLord/rust,avdi/rust,kimroen/rust,zubron/rust,rprichard/rust,zubron/rust,stepancheg/rust-ide-rust,aneeshusa/rust,zubron/rust,sae-bom/rust,omasanori/rust,mdinger/rust,zaeleus/rust,KokaKiwi/rust,kimroen/rust,dinfuehr/rust,emk/rust,richo/rust,0x73/rust,ruud-v-a/rust,andars/rust,kwantam/rust,dinfuehr/rust,kimroen/rust,mitsuhiko/rust,vhbit/rust,reem/rust,hauleth/rust,erickt/rust,ejjeong/rust,aturon/rust,robertg/rust,gifnksm/rust,nham/rust,erickt/rust,stepancheg/rust-ide-rust,jbclements/rust,ktossell/rust,cllns/rust,mahkoh/rust,pczarn/rust,AerialX/rust,GBGamer/rust,SiegeLord/rust,vhbit/rust,jashank/rust,nwin/rust,Ryman/rust,pelmers/rust,philyoon/rust,kwantam/rust,fabricedesre/rust,seanrivera/rust,bombless/rust,mdinger/rust,fabricedesre/rust,nwin/rust,victorvde/rust,zachwick/rust,hauleth/rust,bhickey/rand,KokaKiwi/rust,GBGamer/rust,carols10cents/rust,XMPPwocky/rust,gifnksm/rust,quornian/rust,jroesch/rust,P1start/rust,mitsuhiko/rust,zubron/rust,zubron/rust,l0kod/rust,SiegeLord/rust,fabricedesre/rust,pelmers/rust,servo/rust,stepancheg/rust-ide-rust,aturon/rust,mihneadb/rust,richo/rust,mahkoh/rust,pczarn/rust,stepancheg/rust-ide-rust,rohitjoshi/rust,pczarn/rust,aidancully/rust,mitsuhiko/rust,KokaKiwi/rust,bombless/rust-docs-chinese,quornian/rust,ktossell/rust,GBGamer/rust,pshc/rust,AerialX/rust-rt-minimal,TheNeikos/rust,defuz/rust,fabricedesre/rust,defuz/rust,robertg/rust,richo/rust,ruud-v-a/rust,erickt/rust,rohitjoshi/rust,servo/rust,emk/rust,waynenilsen/rand,ebfull/rust,l0kod/rust,ruud-v-a/rust,vhbit/rust,untitaker/rust,zaeleus/rust,defuz/rust,0x73/rust,P1start/rust,0x73/rust,barosl/rust,Ryman/rust,rohitjoshi/rust,pshc/rust,AerialX/rust,fabricedesre/rust,GBGamer/rust,0x73/rust,cllns/rust,robertg/rust,carols10cents/rust,mahkoh/rust,ejjeong/rust,defuz/rust,pelmers/rust,seanrivera/rust,GBGamer/rust,jbclements/rust,andars/rust,0x73/rust,omasanori/rust,quornian/rust,jbclements/rust,kimroen/rust,mihneadb/rust,Ryman/rust,philyoon/rust,victorvde/rust,nwin/rust,AerialX/rust-rt-minimal,avdi/rust,retep998/rand,pshc/rust,quornian/rust,pelmers/rust,cllns/rust,zachwick/rust,nham/rust,bombless/rust,dwillmer/rust,victorvde/rust,pshc/rust,carols10cents/rust,seanrivera/rust,fabricedesre/rust,KokaKiwi/rust,barosl/rust,ebfull/rust,miniupnp/rust,AerialX/rust,nham/rust,defuz/rust,rprichard/rust,aturon/rust,Ryman/rust,aepsil0n/rust,mdinger/rust,avdi/rust,quornian/rust,ruud-v-a/rust,nwin/rust,miniupnp/rust,servo/rust,ebfull/rust,zubron/rust,bombless/rust,graydon/rust,dwillmer/rust,dwillmer/rust,jroesch/rust,rprichard/rust,emk/rust,jroesch/rust,mihneadb/rust,ktossell/rust,kimroen/rust,ebfull/rust,aidancully/rust,pythonesque/rust,miniupnp/rust,barosl/rust,barosl/rust,P1start/rust,kwantam/rust,dwillmer/rust,aneeshusa/rust,victorvde/rust,erickt/rust,philyoon/rust,mihneadb/rust,krzysz00/rust,ruud-v-a/rust,aturon/rust,mvdnes/rust,TheNeikos/rust,gifnksm/rust,ktossell/rust,untitaker/rust,jashank/rust,ktossell/rust,michaelballantyne/rust-gpu,mdinger/rust,mvdnes/rust,zubron/rust,hauleth/rust,mahkoh/rust,KokaKiwi/rust,dinfuehr/rust,miniupnp/rust,GrahamDennis/rand,jbclements/rust,dinfuehr/rust,jashank/rust,omasanori/rust,ruud-v-a/rust,kimroen/rust,aidancully/rust,andars/rust,aepsil0n/rust,zaeleus/rust,erickt/rust,jashank/rust,carols10cents/rust,michaelballantyne/rust-gpu,pelmers/rust,aturon/rust,aidancully/rust,avdi/rust,graydon/rust,michaelballantyne/rust-gpu,avdi/rust,untitaker/rust,jbclements/rust,achanda/rand,huonw/rand,miniupnp/rust,erickt/rust,emk/rust,emk/rust,graydon/rust,Ryman/rust,AerialX/rust-rt-minimal,pythonesque/rust,dwillmer/rust,omasanori/rust,SiegeLord/rust,l0kod/rust,zaeleus/rust,nham/rust,l0kod/rust,erickt/rust,nham/rust,quornian/rust,cllns/rust