Move the panicking parse functions out of the parser

Since these functions are only used by the AST quoting syntax extensions, they should be there instead of in the parser.

src/test/auxiliary/macro_crate_test.rs

@@ -54,7 +54,7 @@ fn expand_identity(cx: &mut ExtCtxt, _span: Span, tts: &[TokenTree])
     // Parse an expression and emit it unchanged.
     let mut parser = parse::new_parser_from_tts(cx.parse_sess(),
         cx.cfg(), tts.to_vec());
-    let expr = parser.parse_expr_panic();
+    let expr = parser.parse_expr().unwrap();
     MacEager::expr(quote_expr!(&mut *cx, $expr))
 }