Static type checking, and possibly automatic generation of C code

[Changaco]

cbor2 lacks type hints. In addition to enabling static type checking, these annotations might allow mypyc to automatically generate the C version of the module. (Back in #47, using Cython was considered for this but ultimately rejected due to performance concerns. Perhaps mypyc can do better.)

[Sekenre]

Type hints are hopefully coming soon, I would also accept patches.