Find a better mp-int library to handle costly large integer calculation.

[jangko]

Running 9 modexp test of eth test suite, the result is:

geth: 840+ ms
nimbus + tommath: 4s. 760+ ms
nimbus + stint: 12s, 880+ ms

Find a better mp-int library that can handle arbitrary input length and have modpow/modexp optimization.
And not limited to modexp precompile only, maybe can be extended to other area where stint is slow.

[winsvega]

In my experience if you want smth optimised for your particular problem, the best is to do it yourself.

[mratsim]

This implementation offers a 80x speedup over Stint on Uint256 modular exponentiation (and is generic to any runtime size):

mratsim/constantine#242 (comment)