Check first SRS point is on curve when downloading

[ludamad]

We now have bn254 and grumpkin, and they come from internet sources. We don't check the SRS is on curve at all, adding to a checklist of stuff if a proof fails to verify. It will remove domain knowledge needed to debug + make the 'does not verify' debugging checklist simpler if this just checks the first g1/g2 point is on curve (bn254 or grumpkin)