crypto fork: consider a url like golang.tendermint.com/x/crypto

[ebuchman]

Following up on #2558

Currently, the imports use golang.org/x/crypto and we leave comments that we actually forked it. We could make the comments clearer (#2558 (review)), but even better would be a custom URL (#2558 (comment)), though this requires the URL to actually exist else dep will throw.

We should create golang.tendermint.com/x/crypto, redirect it to the godoc, and replace the imports in the code

[liamsi]

I think this was addressed in #3027.
We do not use the fork anymore.