Use AttrVec in more places.

In some places we use `Vec<Attribute>` and some places we use
`ThinVec<Attribute>` (a.k.a. `AttrVec`). This results in various points
where we have to convert between `Vec` and `ThinVec`.

This commit changes the places that use `Vec<Attribute>` to use
`AttrVec`. A lot of this is mechanical and boring, but there are
some interesting parts:
- It adds a few new methods to `ThinVec`.
- It implements `MapInPlace` for `ThinVec`, and introduces a macro to
  avoid the repetition of this trait for `Vec`, `SmallVec`, and
  `ThinVec`.

Overall, it makes the code a little nicer, and has little effect on
performance. But it is a precursor to removing
`rustc_data_structures::thin_vec::ThinVec` and replacing it with
`thin_vec::ThinVec`, which is implemented more efficiently.

compiler/rustc_driver/src/lib.rs

@@ -1070,7 +1070,7 @@ pub fn handle_options(args: &[String]) -> Option<getopts::Matches> {
     Some(matches)
 }
 
-fn parse_crate_attrs<'a>(sess: &'a Session, input: &Input) -> PResult<'a, Vec<ast::Attribute>> {
+fn parse_crate_attrs<'a>(sess: &'a Session, input: &Input) -> PResult<'a, ast::AttrVec> {
     match input {
         Input::File(ifile) => rustc_parse::parse_crate_attrs_from_file(ifile, &sess.parse_sess),
         Input::Str { name, input } => rustc_parse::parse_crate_attrs_from_source_str(