std::trie: optimise insert slightly.
std::trie: optimise insert slightly.

This reduces the number of moves/memcpy's we do, which makes insert
faster, especially in cases of keys with long equal prefixes (the
_low_bits tests):

Before:

    bench_insert_large                ... bench:    553966 ns/iter (+/- 64050)
    bench_insert_large_low_bits       ... bench:   1048151 ns/iter (+/- 92484)
    bench_insert_small                ... bench:    168840 ns/iter (+/- 22410)
    bench_insert_small_low_bits       ... bench:    185069 ns/iter (+/- 38332)

After:

    bench_insert_large                ... bench:    422132 ns/iter (+/- 35112)
    bench_insert_large_low_bits       ... bench:    339083 ns/iter (+/- 34421)
    bench_insert_small                ... bench:    134539 ns/iter (+/- 15254)
    bench_insert_small_low_bits       ... bench:     88775 ns/iter (+/- 5746)

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