ecc: MultiExp code clean up

[gbotrel]

Different code paths exists to perform scalar multiplications:
ScalarMul, ScalarMulByGen, WindowedMultiExp and MultiExp . Adding to the internal methods multiExp and pippenger .

1. Current MultiExp doesn't scale as well as it could with available CPUs and number of constraints.
2. The Setup would benefit from having a fixed based MultiExp (see pippenger algorithm).
3. The ScalarMul would likely benefit from taking advantage of the endomorphism described in Pairing for beginners

Once these are addressed, we need to clean up the APIs, then would be a good time to extract the ecc package from gnark.