Implement ToSource and ToToken for ast::Arg

This makes ast::Arg usable in the quote_ macros.

Please note that this commit doesn't include a regression test. There
are tests that use the quote macros, but all of them are ignored. Due to
that, there is no obvious (to me) way to test this.

Since this change is absolutely trivial and only hooks up an additional
type to existing infrastructure (which presumably is tested elsewhere),
I concluded it's not worth the effort to follow up on this.
auto merge of #14722 : hannobraun/rust/quote-args, r=alexcrichton

The need for this came up while I worked on a procedural macro that parses an argument and adds the same argument to a generated function.