Get test-pass/utf8.rs to run

This involved a small fix to the unicode-escape character lexing and
to the pretty-printer.

src/comp/pretty/pprust.rs

@@ -957,7 +957,8 @@ fn escape_str(str st, char to_escape) -> str {
             case ('\\') {out += "\\\\";}
             case (?cur) {
                 if (cur == to_escape) {out += "\\";}
-                str::push_byte(out, cur as u8);
+                // FIXME some (or all?) non-ascii things should be escaped
+                str::push_char(out, cur);
             }
         }
         i += 1u;