Implement vectorised Committable and Updatable in ipa-multipoint

[thomas-quadratic]

Currently, operations in Committable and Updatable are unitary: they produce a single commitment.
To avoid multiple context switching between java and rust, we would like to be able to commit or update multiple commitments at a time.

All operations in Committable and Updatable traits should therefore include a _vec version (e.g. commit_vec) that maps the corresponding operation over a Vec.

The vectorised versions should also have entrypoints for JNI with RLP codec.
The implementation should come with tests for all the new vectorised operations.