Important bug fix in fast version

This update switches Poly1305 fast version back to original (slow) version.

There was a bug in fast version of Poly1305 which sometimes produced incorrect results. Authenticity of results of the following functions may be affected: secretbox, secretbox.open, box, box.open.

Fast version is default if you used npm package (e.g. require("tweetnacl")).

TweetNaCl.js comes in two favors: nacl.js, which is almost the exact port of TweetNaCl, and nacl-fast.js, which includes faster versions ported from other implementations. The fast version of Poly1305 message authenticator comes from 16-bit version of floodyberry/poly1305-donna. The bug is present in this version and was ported to JavaScript.

Until we figure how to fix the bug, Poly1305 in nacl-fast.js was switched to original nacl.js.