C implementation of 'warnings' now in the trunk

I finally finished the re-implementation of the 'warnings' module in C. This allows for a couple of things. One is that now warnings can be emitted by the parser since the module is built-in. Two, hopefully this can speed some things up like interpreter start-up if the C version of the module is used over the pure Python implementation. And third, this was needed by me for importlib to deal with bootstrapping.