Reentrancy in EC G1 multiplication

[gballet]

While investigating this issue in our verkle tree library, we figured out that there is a re-entrancy issue in EC point multiplication. A description and a test to reproduce the problem can be found here.

We are aware that the README states that group instances and pairing instances are not suitable for concurrent contexts. This issue is here to 1) declare interest in a goroutine-safe version of this library 2) to document that issue in case someone else runs into it.