Create a benchmarking V-App and test suite

[bigspider]

It would be great to have a test V-app and an associated benchmarking test suite to run some computations of varying complexity and requirements on Vanadium (on a real device), in order to measure the performance and assess future improvements.

Example of computations that we could include:

• base58 encode/decode
• sha256 hashes
• maybe some heavier stuff like ECDSA/Schnorr signatures (without ECALLs)
• Sieve of Eratosthenes to count primes up to a threshold (memory intensive)
• Something that sends some buffers back and forth with no computations (to stress the communication protocol)

That could also be a good way of testing the CPU until #19 is addressed, so it's worth testing the results of those computations on Speculos as part of the CI.