Use wrapper structs for `BTreeSet`'s iterators.
Use wrapper structs for `BTreeSet`'s iterators.

Using a type alias for iterator implementations is fragile since this
exposes the implementation to users of the iterator, and any changes
could break existing code.

This commit changes the iterators of `BTreeSet` to use
proper new types, rather than type aliases.  However, since it is
fair-game to treat a type-alias as the aliased type, this is a:

[breaking-change].

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