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/test.rs

@@ -125,7 +125,7 @@ impl<'a> fold::Folder for TestHarnessGenerator<'a> {
             match i.node {
                 ast::ItemFn(_, ast::Unsafety::Unsafe, _, _, _) => {
                     let diag = self.cx.span_diagnostic;
-                    diag.span_fatal(i.span, "unsafe functions cannot be used for tests");
+                    panic!(diag.span_fatal(i.span, "unsafe functions cannot be used for tests"));
                 }
                 _ => {
                     debug!("this is a test function");