Better multithreading

[ihaveamac]

Certain parts of pyctr could probably benefit from threading support. One idea I have is to split file reading and de/encryption into two threads with their own locks; since this is mostly IO-bound it should be fine with the GIL (and pycryptodome releases the GIL when doing crypto work).

Per file:

1. Lock file-reading thread
2. Read from file (CIA, NCCH, etc.)
3. Unlock file-reading thread
4. Pass to decrypt thread
5. Lock decrypt thread
6. Decrypt data
7. Unlock decrypt thread
8. Return data to main thread

This would allow a file read operation to continue at the same time decryption is happening for another part of the file.

(connected to: ihaveamac/ninfs#94)