gccrs: Improve method resolution to use seperate indexes for impl's gcc/rust/ChangeLog: * typecheck/rust-hir-dot-operator.cc (MethodResolver::select): use sperate index * util/rust-hir-map.cc (Mappings::insert_hir_impl_block): new indexes * util/rust-hir-map.h: likewise Signed-off-by: Philip Herron <herron.philip@googlemail.com>