Tails and accumulator refactoring

[andrewwhitehead]

Adds wrappers around the Accumulator and Tail types instead of exposing PointG2 (no compatiblity change for indy-credx or anoncreds-rs).

Adds utility methods to RevocationRegistry for generating the initial state and a future state with a set of issued indexes, for use in anoncreds-rs.

[swcurran]

What is the purpose of the new utility methods? Are they to make it easier to handle full state vs. delta handling? Thanks

[andrewwhitehead]

@swcurran Yes essentially, at the moment anoncreds-rs creates an empty registry and then uses update_revocation_registry which is more complicated and less efficient.