remove all instances of mt_rand

Similar to #1, I've removed `mt_rand()` and other functions that depend on it, which aren't needed for keypair generation.  I didn't want anyone to see the extra functionality available and use it without knowing it was weak.

I recognize this is an awkward patch in that it changes minified upstream code, making it hard to audit.  I'm open to other ways to proceed, and am also not as concerned about this as I am about #1.  Thanks.