[feat] Precompile MODEXP

[noel2004]

Work plan:

• Induce and wrap modexp circuit from Gaoxin's work
• Parse memory bytes for input and connect with the wrapped modexp circuit via ModExpTable
• Error handling: have to prove any invalid input bytes lead to empty output (the precompile always has a successful call)

This work would base on the PR of ECRECOVER to make use of some essential feature

[DO NOT MERGE] until following has completed:

• Include Re-use cells between mutually exclusive conditions #601 so we can enable both ecrecover and modexp
• adjust to the behavior identical with l2geth (only len=32 is valid and return error in invalid input)

Pending tasks (not planned in current PR)

• truncate / zero padding the input bytes according to expected input size
• gas cost
• pass a mock prove

[lispc]

i noticed so many codes is used to handle padding bytes..

[noel2004]

i noticed so many codes is used to handle padding bytes..

Yes. Some code is written before PowOfRand table can be used. Now they have been pruned.