auto merge of #19248 : japaric/rust/str, r=alexcrichton
auto merge of #19248 : japaric/rust/str, r=alexcrichton

Just like we do with AsSlice

This comes in handy when dealing with iterator-centric APIs (`IntoIterator`!) and you want to receive an `Iterator<S> where S: Str` argument. Without this PR, e.g. you can't receive `&["a", "b"].iter()` instead you'll have to type `&["a", "b"].iter().map(|&x| x)` (A similar thing happens with `&[String]`).

r? @aturon 

Full disclaimer: I haven't run `make`/`make check` yet (All my cores are busy)

bombless/rust,l0kod/rust,mdinger/rust,ktossell/rust,ruud-v-a/rust,quornian/rust,carols10cents/rust,kwantam/rust,omasanori/rust,seanrivera/rust,miniupnp/rust,miniupnp/rust,GBGamer/rust,mihneadb/rust,nwin/rust,jroesch/rust,avdi/rust,mahkoh/rust,hauleth/rust,pelmers/rust,untitaker/rust,avdi/rust,zaeleus/rust,miniupnp/rust,krzysz00/rust,ruud-v-a/rust,richo/rust,rprichard/rust,nwin/rust,miniupnp/rust,hauleth/rust,zubron/rust,andars/rust,ktossell/rust,nwin/rust,XMPPwocky/rust,ebfull/rust,nwin/rust,ejjeong/rust,nwin/rust,hauleth/rust,emk/rust,aneeshusa/rust,rohitjoshi/rust,pshc/rust,victorvde/rust,TheNeikos/rust,seanrivera/rust,kwantam/rust,dwillmer/rust,emk/rust,sae-bom/rust,dinfuehr/rust,KokaKiwi/rust,defuz/rust,ruud-v-a/rust,TheNeikos/rust,sae-bom/rust,carols10cents/rust,AerialX/rust-rt-minimal,ebfull/rust,AerialX/rust,andars/rust,cllns/rust,nwin/rust,kwantam/rust,bombless/rust,ruud-v-a/rust,aidancully/rust,dwillmer/rust,emk/rust,AerialX/rust-rt-minimal,carols10cents/rust,AerialX/rust,l0kod/rust,krzysz00/rust,untitaker/rust,aneeshusa/rust,pshc/rust,victorvde/rust,jashank/rust,krzysz00/rust,andars/rust,pshc/rust,quornian/rust,zachwick/rust,miniupnp/rust,bombless/rust,zubron/rust,zubron/rust,andars/rust,zaeleus/rust,jroesch/rust,mvdnes/rust,gifnksm/rust,kimroen/rust,kimroen/rust,pshc/rust,sae-bom/rust,zubron/rust,l0kod/rust,reem/rust,victorvde/rust,vhbit/rust,philyoon/rust,mihneadb/rust,l0kod/rust,jashank/rust,AerialX/rust,ejjeong/rust,GBGamer/rust,gifnksm/rust,pelmers/rust,carols10cents/rust,seanrivera/rust,mahkoh/rust,GBGamer/rust,XMPPwocky/rust,KokaKiwi/rust,krzysz00/rust,zubron/rust,quornian/rust,emk/rust,gifnksm/rust,ebfull/rust,GBGamer/rust,philyoon/rust,mdinger/rust,mahkoh/rust,AerialX/rust-rt-minimal,XMPPwocky/rust,omasanori/rust,robertg/rust,seanrivera/rust,kimroen/rust,richo/rust,zubron/rust,reem/rust,dwillmer/rust,aepsil0n/rust,mvdnes/rust,gifnksm/rust,mahkoh/rust,graydon/rust,jroesch/rust,mvdnes/rust,cllns/rust,philyoon/rust,ktossell/rust,ktossell/rust,vhbit/rust,pelmers/rust,zachwick/rust,TheNeikos/rust,pelmers/rust,dinfuehr/rust,rohitjoshi/rust,philyoon/rust,KokaKiwi/rust,mihneadb/rust,bombless/rust-docs-chinese,kimroen/rust,vhbit/rust,sae-bom/rust,omasanori/rust,defuz/rust,aepsil0n/rust,AerialX/rust,avdi/rust,ruud-v-a/rust,jroesch/rust,untitaker/rust,aidancully/rust,rprichard/rust,jroesch/rust,ktossell/rust,mdinger/rust,l0kod/rust,mvdnes/rust,ejjeong/rust,mihneadb/rust,mihneadb/rust,richo/rust,zubron/rust,richo/rust,dwillmer/rust,vhbit/rust,zachwick/rust,jashank/rust,nwin/rust,pshc/rust,avdi/rust,rohitjoshi/rust,hauleth/rust,jroesch/rust,seanrivera/rust,jashank/rust,aidancully/rust,sae-bom/rust,AerialX/rust-rt-minimal,KokaKiwi/rust,GBGamer/rust,GBGamer/rust,zubron/rust,zachwick/rust,ejjeong/rust,miniupnp/rust,bombless/rust,ebfull/rust,kimroen/rust,pshc/rust,XMPPwocky/rust,mvdnes/rust,AerialX/rust,ebfull/rust,jashank/rust,untitaker/rust,miniupnp/rust,rohitjoshi/rust,zachwick/rust,dinfuehr/rust,graydon/rust,ebfull/rust,quornian/rust,mvdnes/rust,richo/rust,aidancully/rust,victorvde/rust,graydon/rust,hauleth/rust,pelmers/rust,dwillmer/rust,vhbit/rust,carols10cents/rust,quornian/rust,mdinger/rust,kimroen/rust,andars/rust,mdinger/rust,reem/rust,aneeshusa/rust,rprichard/rust,emk/rust,ejjeong/rust,l0kod/rust,andars/rust,cllns/rust,AerialX/rust-rt-minimal,ruud-v-a/rust,cllns/rust,jashank/rust,TheNeikos/rust,quornian/rust,ktossell/rust,kwantam/rust,krzysz00/rust,jashank/rust,emk/rust,dinfuehr/rust,victorvde/rust,pshc/rust,dwillmer/rust,hauleth/rust,gifnksm/rust,TheNeikos/rust,rohitjoshi/rust,graydon/rust,graydon/rust,l0kod/rust,aidancully/rust,pshc/rust,omasanori/rust,sae-bom/rust,vhbit/rust,rprichard/rust,ktossell/rust,philyoon/rust,reem/rust,zaeleus/rust,robertg/rust,aepsil0n/rust,dwillmer/rust,gifnksm/rust,victorvde/rust,untitaker/rust,AerialX/rust-rt-minimal,kwantam/rust,aneeshusa/rust,kwantam/rust,reem/rust,rohitjoshi/rust,aepsil0n/rust,TheNeikos/rust,robertg/rust,dinfuehr/rust,pelmers/rust,seanrivera/rust,mahkoh/rust,zachwick/rust,KokaKiwi/rust,defuz/rust,reem/rust,aepsil0n/rust,rprichard/rust,AerialX/rust,XMPPwocky/rust,jroesch/rust,defuz/rust,philyoon/rust,GBGamer/rust,aepsil0n/rust,avdi/rust,XMPPwocky/rust,robertg/rust,quornian/rust,untitaker/rust,zaeleus/rust,dinfuehr/rust,robertg/rust,KokaKiwi/rust,mihneadb/rust,dwillmer/rust,aidancully/rust,defuz/rust,cllns/rust,aneeshusa/rust,l0kod/rust,zaeleus/rust,aneeshusa/rust,GBGamer/rust,richo/rust,ejjeong/rust,cllns/rust,graydon/rust,krzysz00/rust,omasanori/rust,jroesch/rust,robertg/rust,avdi/rust,omasanori/rust,carols10cents/rust,jashank/rust,vhbit/rust,miniupnp/rust,zaeleus/rust,bombless/rust,mdinger/rust,mahkoh/rust,nwin/rust,vhbit/rust,emk/rust,bombless/rust,defuz/rust,kimroen/rust,rprichard/rust