CodeObject wrapper and global CodeObject registry

[tzanko-matev]

Every event callback receives a _code argument which is of the Python type CodeObject. We created a CodeObjectWrapper type which wraps this object for two purposes:

1. To expose an easier to use interface which would focus on what we need to implement a tracer
2. To minimize performance hit by caching the code object data that we fetch from the Python object.

A code object corresponds to the body of a function or a module. With our wrapper and registry we guarantee that each code object is processed at most once.