equix: API changes for new result codes and hashx compatibility

This change adapts Equi-X to the corresponding HashX API changes that
added HASHX_TRY_COMPILE. The new regularized HashX return codes are
reflected by revised corresponding Equi-X return codes.

Both solve and verify operations now return an error/success code, and a
new equix_solutions_buffer struct includes both the solution buffer
and information about the solution count and hashx implementation.

With this change, it's possible to discern between hash construction
failures (invalid seed) and some external error like an mprotect()
failure.

Signed-off-by: Micah Elizabeth Scott <beth@torproject.org>