Made receivers on hkex.Conn mutators *Conn again (whoops)
TODO: Consider: padding (? probably not, XORKeyStream OFB/CBC/etc. modes prevent
constant header/crib exposure, and would add lots of complexity to Read/Write)
TODO: Add CTR, other modes
-Note about blowfish iv len (lack of) bounds check in .NewOFB();
-TODO added to enforce keymat from HKex >= 2*chosen cipher blocksize
(assuming keylen == blocksize -- might not be true for all future algs)