Multiple functions should be(and aren't) using BigInteger

[dapinkone]

Multiple functions should be (and aren't) using BigInteger. We were able to skirt this requirement for part 1, but it looks like it's a hard requirement for part 2 with edwards curves.

• bytepad
• left_encode
• cSHAKE256(N and S)
• KMACSOF256
• others?