Bump version to 0.1.0.1
Add cabal sandbox to .gitignore

Also stop ignoring scattered .hi and .o files, they should all be in
dist/ anyway.
Use Data.Atomics for safe compare-and-swap

This obviates the need for reallyUnsafePtrEquality# and also appears to
be slightly faster (it uses GHC's new native CAS operations).

Thanks to @gregorycollins for pointing me in this direction.