Refactor SRS module to support SRS on both G1 and G2
This MR refactors the Srs module to support SRS on the curves G1 and G2:
- srs functor parametrized by the curve type
- separate srs stubs from polynomial stubs
- we use the inplace FFT on G1 and G2 from ocaml-bls12-381 rewritten in C
- we add the srs size to the filename and keep its source (zcash or filecoin) in the filename
Edited by Marco Stronati