[Keyless Common] Various cleanups, refactors and improvements to the keyless-common directory.

[JoshLind]

What is the change being pushed?

This PR offers the following commits:

1. Clean up bits.rs (and add a simple test), and split groth16_vk.rs into two files (snark_js_groth16.rs and types.rs).
2. Rename config.rs -> circuit_config.rs.
3. Clean up circuit_config.rs and harden file loading using serde.
4. Move the test witness generation logic into the circuit directory, and add simple tests to the circuit config code.
5. Clean up sha.rs and remove the code duplication between compute_sha_padding and compute_sha_padding_without_length. Now, a flag determines if the length should be included.
6. Split up encoding.rs, remove the JWT related content (and create a jwt.rs file), and move trait implementations to appropriate places.
7. Clean up circuit_input_signals.rs.

Note: other than cleanups, refactors and reorganization, nothing should logically change.

Testing Plan

New and existing test infrastructure.