Make the parser more careful about keywords

Keywords are now only recognized in contexts where they are valid. The
lexer no longer recognizes them, all words are lexed as IDENT tokens,
that get interpreted by the parser.

src/lib/std.rc

@@ -51,7 +51,7 @@ alt (target_os) {
     } case ("macos") {
         mod os = "macos_os.rs";
         mod os_fs = "posix_fs.rs";
-    } else {
+    } case (_) {
         mod os = "linux_os.rs";
         mod os_fs = "posix_fs.rs";
     }