clean up the repo a bit
Summary:
Moved our implementation of the opaque-ke CipherSuite trait to a separate library to avoid repeating ourselves.
Deleted opaque-ke-cxx which we didn't end up using.
Updated .gitignore files.
Test Plan: rebuilt everything that I refactored to use the new comm-opaque library and it worked fine
Reviewers: jon, tomek
Reviewed By: jon, tomek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D5880