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/parse/mod.rs

@@ -36,9 +36,6 @@ pub mod attr;
 /// Common routines shared by parser mods
 pub mod common;
 
-/// Functions dealing with operator precedence
-pub mod prec;
-
 /// Routines the parser uses to classify AST nodes
 pub mod classify;