libsyntax: Fix snake_case errors.

A number of functions/methods have been moved or renamed to align
better with rust standard conventions.

syntax::ext::mtwt::xorPush => xor_push
syntax::parse::parser::Parser => Parser::new

[breaking-change]

src/libsyntax/ast.rs

@@ -26,6 +26,7 @@ use serialize::{Encodable, Decodable, Encoder, Decoder};
 /// A pointer abstraction. FIXME(eddyb) #10676 use Rc<T> in the future.
 pub type P<T> = @T;
 
+#[allow(non_snake_case_functions)]
 /// Construct a P<T> from a T value.
 pub fn P<T: 'static>(value: T) -> P<T> {
     @value