Export cava2 library in a few prepackaged files

[jadephilipoom]

See discussion in #929

The previous cava library had two main files that library users would import, Cava.Cava (imports needed for circuit definitions and tests) and Cava.CavaProperties (imports needed for proofs), which reduced import complexity quite a bit. It's probably a good idea to replicate that structure in cava2, and would also help us better control tvar typeclass inference as discussed in #929.