This PR improves the IsOnCurve methods for BN254 G2 points by:
* Clarifying its behavior the docstring, making it explicit that it
verifies both the point being on the curve and in the correct subgroup.
* Adding an in-line comment explaining the subgroup membership check
(c.Mul(Order)).
* Minor wording adjustments for readability and consistency.
Adds marshaling fuzzing for G1 and G2 to oss-fuzz.
Also aligns the behavior of the google library to that of gnark and
cloudflare, which only ever read the first 64 / 128 bytes of the input,
regardless of how long the input is
* core/vm, crypto/bn256: switch over to cloudflare library
* crypto/bn256: unmarshal constraint + start pure go impl
* crypto/bn256: combo cloudflare and google lib
* travis: drop 386 test job
