Eliminate a copy in syntax::parse::new_parser_from_file

Fixing a FIXME turned out to be pretty involved. I added an io function
that returns a unique boxed string (for the contents of a file) rather than
a string, and went from there. Also made the src field of codemap a unique
boxed string. This doesn't seem to make that much difference in amount of
allocation according to valgrind (disappointingly), but I also had to introduce
a copy somewhere else pending a new snapshot, so maybe that's it.

src/rustc/driver/driver.rs

@@ -78,7 +78,7 @@ fn parse_input(sess: session, cfg: ast::crate_cfg, input: str)
     if !input_is_stdin(input) {
         parse::parse_crate_from_file(input, cfg, sess.parse_sess)
     } else {
-        let src = @str::from_bytes(io::stdin().read_whole_stream());
+        let src = ~str::from_bytes(io::stdin().read_whole_stream());
         parse::parse_crate_from_source_str(input, src, cfg, sess.parse_sess)
     }
 }