fix modexp precompile to handle arbitrary sized input.

[jangko]

From modexp spec:
"Calculates (base**exp) % modulus for arbitrary sized base, exp and. modulus. The return value is the same length as the modulus."

Current situation:
modexp is implemented using Stint and only able to handle fixed size integer up to 1024 bytes(StUint[8192]).
Nimbus will refuse to calculate modexp when one of the input size is longer than 1024.

Possible solution:
Integrate an additional BigInt library to be used specifically for this operation or implement a one-off algorithm for it.