Work towards a non-panicing parser (libsyntax)

- Functions in parser.rs return PResult<> rather than panicing
- Other functions in libsyntax call panic! explicitly for now if they rely on panicing behaviour.
- 'panictry!' macro added as scaffolding while converting panicing functions.
  (This does the same as 'unwrap()' but is easier to grep for and turn into try!())
- Leaves panicing wrappers for the following functions so that the
  quote_* macros behave the same:
  - parse_expr, parse_item, parse_pat, parse_arm, parse_ty, parse_stmt

src/test/run-pass-fulldeps/quote-tokens.rs

@@ -17,6 +17,7 @@ extern crate syntax;
 
 use syntax::ext::base::ExtCtxt;
 use syntax::ptr::P;
+use syntax::parse::PResult;
 
 fn syntax_extension(cx: &ExtCtxt) {
     let e_toks : Vec<syntax::ast::TokenTree> = quote_tokens!(cx, 1 + 2);