tor-proto: Add benchmarks for cell encryption and decryption

This code sets up benchmarks for thetor-proto crate using criterion and adds benchmarks of the cell encryption and decryption functions as well as the is_recognized and set_digest functions.

The benchmarks for those functions are generic enough to be adapted to benchmark future cryptographic schemes like CGO (see proposal 308) or other relay cell formats like the one used in the proposal 340.

It is the code that I used to generate the results for the issue #1336.