libsyntax: add &self to parser methods
libsyntax: add &self to extensions
libsyntax: deny deprecated_self in libsyntax
auto merge of #5207 : erickt/rust/libsyntax-deprecated-self, r=brson

This adds an explicit `&self` to each method in libsyntax, and sets the `#[deny(deprecated_self)]` in `syntax.rc`.