App circuit should generate read requests, kernel should validate them

[dbanks12]

Original discussion: https://discourse.aztec.network/t/to-read-or-not-to-read/178/10
Relevant discussion outlining the tasks below: https://discourse.aztec.network/t/spending-notes-which-havent-yet-been-inserted/180/4

Blocks #512

1. Add READ_REQUESTS_LENGTH, update PrivateCircuitPublicInputs with read_requests
2. Update PrivateCallData with read_request_membership_witnesses which includes leaf indexes and sibling paths
3. Update Noir's get() to no longer do anything related to membership checks (omit leaf indexes and sibling paths in return)
   • implications on public kernel?
4. Update Noir's get() to stop populating historic_private_data_tree_root
5. Update kernel to loop through each read_request and use the corresponding read_request_membership_witness to compute root of data tree
   • this root must be the same for every iteration in this loop
   • the root can then be set equal to the private_kernel_public_inputs.constant_data.old_tree_roots.private_data_tree_root