These are: region,arg,fn_decl,method,_mod,foreign_mod,
variant_arg,enum_def_,variant_,trait_ref.

As per https://github.com/mozilla/rust/pull/4253

This reverts commit 6c056976.

add a Set trait

repr: represent char values as char literals rather than integers.

Fix the difference method on bit vectors

Prevents a case of "unknown scope, this is a bug".

I judged this too small to review. Closes #4500

After a snapshot, we can take out the set_buffer_ version (pipes
syntax exts use it).

As per #4421

Cuts down on bad copies (though there are still a few that can go
away once there are impls of push / append / + for @str)

rs=breakage

r=graydon

def-id we have, so that the inliner can distinguish between external and
internal def-ids.  Also add some comments explaining the distinction!

Fixes #4516.

r=graydon