[WIP] [BigInt] Add impl & tests

[nrryuya]

code size reduction

For code size reduction, I inlined these functions in the previous code,

• _convertUInt256ListToBytes
• _convertBytesArrayToUInt256List
• _compare

following the changes made in #72 from the first commit to the third commit.

What's next?

• Add isPrime.vy (#74)
• Fix RSAAccumulator.vy to use this and isPrime.vy

Others

• Should we divide the result of modularMul4 and modularMulBy4 by 4?
• Is it more efficient (in gas or code size) to do arithmetic in bytes[256] (without converting them to uint256[8]?) like other implementations?
  • zcoinofficial/solidity-BigNumber
  • axic/BigInt.sol