secp256k1 and sha256_extend mocks + refactor

[mcalancea]

• all precompiles have the same interface as their counter-parts in sp1; the secp256k1 curve ops are backed by the secp crate.
• integrates appropriate dummy circuits for precompiles into the zkvm
• added utils.rs with some utilities for safely manipulating memory segments of the VM
• tested the compatibility of the syscalls (+ keccak, done previously) with sp1 (see the syscalls.rs example file).
• extended the docs pasted from sp1

Aurel has reviewed an earlier version of this here.