#ast should only generate calls to the qquote module

[brson]

The #ast extension generates code that makes calls to internal rustc APIs. Most of those calls are to functions in the qquote module itself, but some are to the parser. This makes changing these parser functions difficult, since the stage0 compiler will continue generating calls to the old function. Code generated by #ast should use an obvious, qquote-specific API, and preferably shouldn't mention any types that exist outside of the qquote module.

For example, the types and functions used by #fmt-generated code all live in std::extfmt::rt.

[graydon]

the replacement (quote.rs) code for this routes everything through syntax::ext::quote::rt. Closing.