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/libsyntax/ext/source_util.rs

@@ -117,11 +117,11 @@ pub fn expand_include<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[ast::TokenTree
             while self.p.token != token::Eof {
                 match self.p.parse_item() {
                     Some(item) => ret.push(item),
-                    None => self.p.span_fatal(
+                    None => panic!(self.p.span_fatal(
                         self.p.span,
                         &format!("expected item, found `{}`",
                                  self.p.this_token_to_string())
-                    )
+                    ))
                 }
             }
             Some(ret)