Get commitments fully working.

[laudiacay]

your todo list for success:

• implement the kate_batch_open and test_kate_open tests in src/plonk/proof_system/commitment_scheme.rs
• come up with a proposal (and implement it) to fix the locks on line 370 of the same file (I suggest locking everything once across all parallel threads then doing something unsafe)
• get the tests passing :)

the kate_batch_open test will require getting transcripts working (and, therefore, pedersen)- as of sept 30, pedersen is unimplemented, and transcripts are untested. so this may get half-done before transcripts and half-done after.

now's a fine time to start, though- this should not be difficult at all if transcripts + pedersen are working.