Multithreaded TinyRemapper#apply

[Devan-Kerman]

The singlethreaded-ness of TinyRemapper#apply is a sore point in my code at the moment. There are 2 issues with the current system that the following pull request fixes:

• The consumer is called on the main thread only
• Writing the classes is not done in the same step as calling the consumer due to limitations in the API surface

I fix this by

• Passing the input tags of a class into the consumer method, making it much easier to use apply and write every file at once.
• Multithreading the apply method