Clean up hash to curve branch and merge it with master

[drskalman]

Improve the quality of the code after removing std from part of library after which we should be able to merge the branch into master.

[burdges]

Arkworks' hash-to-curve interface seems kinda fucked too by not exposing XoFs correctly. I've started a PR but it'll stay impressionistic and need work by someone else. Also this duplicates #5 I think.