Return Python enums from _pygit2

[jorio]

Following up on #1251: C functions in _pygit2 now return actual Python enums instead of ints.

For example, Repository.status() would previously return simple ints. Now it returns FileStatus which transparently demotes to ints for legacy code:

>>> repo.status()
{'file1.txt': <FileStatus.CONFLICTED: 32768>,
 'file2.txt': <FileStatus.INDEX_MODIFIED|WT_MODIFIED: 258>}

Implementation details: References to the enums are cached via cache_modules so that lookups are faster. The references are decref'd in free_module (new module destructor function)

Affected functions:

• Repository.status
• Repository.status_file
• Repository.merge_analysis
• DiffFile.flags
• DiffFile.mode
• DiffDelta.flags
• DiffDelta.status