libsyntax: Introduce an `InternedString` type to reduce `@str` in the
compiler and use it for attributes
libsyntax: De-`@str` literal strings in the AST
libsyntax: De-`@str` `to_source`
librustc: Fix merge fallout.
libsyntax: Remove the unnecessary `src` field from the lexer
librustc: Stop using `@str` for source.
libsyntax: De-`@str` pathnames
libsyntax: Make float literals not use `@str`
libsyntax: Remove all `@str` from the AST
libsyntax: De-`@str` `get_single_str_from_tts`
libsyntax: Remove `ident_to_str` from the parser, which was returning
`@str` values
librustc: Fix merge fallout.
librustc: De-`@str` `NameAndSpan`
libsyntax: De-`@str` `MacroDef`
libsyntax: Fix tests.
librustc: Remove a bunch of `@str` from the compiler around metadata
handling
librustc: De-`@str` resolve
librustc: De-`@str` debuginfo
librustc: De-`@str` symbol hashing
librustc: De-`@str` liveness