libsyntax: De-`@mut` `CodeMap::files`
libsyntax: De-`@mut` `Interner::vect`
libsyntax: De-`@mut` (and de-`@`) `Printer::print_stack`
libsyntax: De-`@mut` `ps::boxes`
FIX for IdentRenamer
librustc: Eliminate an unnecessary `@mut` in pretty printing.

This removes all `@mut` from `librustc` and `libsyntax`.
FIX for codemap files
FIX for interner vect
librustpkg: De-`@mut` `rustpkg`
librustdoc: De-`@mut` `librustdoc`