rustc: minor cleanup
rustc: Remove ty::arg
rustc: rename some ty_self variables to self_arg
syntax: add IterBytes impls for some ast types
rustc: rename a method receiver type to rcvr_ty
rustc: rename ast::self_ty and related fields to explicit_self
rustc: rename ty::method to ty::Method and add ctor
auto merge of #6224 : erickt/rust/rustc-cleanup, r=erickt

Just a couple minor cleanups and renames of librustc