refactor Enum code generation

Summary:
Changed the way how enums generated in thrift-py3:

1. Use C++ map to store name to unique Enum Python instances cache
2. Dynamically find instances from cache (or create one and add to cache) instead of populating all instances at module load time
3. use `__getattr__` and `__getitem__` instead of code-gen all the getters

Reviewed By: asp2insp

Differential Revision: D23182622

fbshipit-source-id: 75baef997ee2d604b4ef7606f282cfc3306208ac

folly/python/range.pxd

@@ -3,6 +3,8 @@ cdef extern from "folly/Range.h" namespace "folly":
         Range()
         Range(T, int)
         T data()
+        T begin()
+        T end()
         int size()
 
 ctypedef Range[const char*] StringPiece