Extended feats

[davxy]

This is the additional stuff we currently use in JAM,

• User provides the padding point. Consequently I removed "try and increment" h2c as it is dead code now
  • just use a random point for tests
• test_vectors feature: Use zeros for ZK rows to easily reproduce test vectors without ugly hacks
• Use ArkTranscript by default: is should be ok for most of use cases and dependency is already not optional
• API consistence: RingVerifier::verify_ring_proof rename to RingVerifier::verify (as we have RingProver::prove)
• Add repo reference (Used by crates.io / docs.rs)

NOTES:

• You may want to merge this first: Add repository reference fflonk#44
• Once this PR has been merged to master, please wait to release it on crates.io. I would like to give it an additional run.

Ty

[drskalman]

LGTM.