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