get rid of prec.rs

prec.rs no longer had much to do with precedence; the token->binop
function fits better in token.rs, and the one-liner defining the
precedence of 'as' can go next to the other precedence stuff in
ast_util.rs

src/libsyntax/ast_util.rs

@@ -355,6 +355,10 @@ pub fn operator_prec(op: ast::binop) -> uint {
   }
 }
 
+/// Precedence of the `as` operator, which is a binary operator
+/// not appearing in the prior table.
+pub static as_prec: uint = 11u;
+
 pub fn dtor_ty() -> @ast::Ty {
     @ast::Ty {id: 0, node: ty_nil, span: dummy_sp()}
 }