[C++] Caller of private kernel can submit "transient" read_requests for "pending" commitments

[dbanks12]

Kernel needs to match transient reads to commitments.

1. write C++ logic to iterate over new_commitments and check for matches when a transient read is encountered
2. add TODO with ticket for optimization where kernel does not iterate in-circuit but instead gets a hint from TS/C++-helper that informs it of the i and j such that we can constrain read_requests[i] == new_commitments[j].
3. write test that read requests work for commitments that don't exist yet in tree

**Note: do not worry about nullifiers as part of this ticket, although this may lead to e2e failures (EDIT: note sure if this is true actually) in which case this ticket can share 1 PR to master with #836