Ensure that type replacements are threadsafe

[al2o3cr]

Modifying a Hash from multiple threads is potentially dangerous, even if threads don't access the same key.

To avoid this, make Mocktail::TopShelf a thread-local singleton.

The list of type replacements corresponds to global changes, so those are represented as a class variable and wrapped in a Mutex.

[searls]

Looks fantastic, this is a huge improvemnt—thank you!!

[searls]

landed in 0.0.5