Use wrapper structs for `HashSet`'s iterators.
Use wrapper structs for `HashSet`'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 `HashSet` 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].

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