Update implementation when standard compliant elligator2 is available in curve25519-dalek

Current implementation of `from_hash` does not correspond to that defined in the standard. Waiting for dalek-cryptography/curve25519-dalek#377

Start by reviewing the current `from_hash` implementation and compare it with the relevant standard definition. The issue says this work is waiting for dalek-cryptography/curve25519-dalek#377; once that dependency is available, update the implementation and verify that its behavior matches the standard.

Written by the indexing model from the issue text.