Add function to return "first_free_address" of ND-memory

[Sword-Smith]

I saw this pattern in stark_verify and thought we might as well have the as a helper function in the memory module.

        let first_free_address = nondeterminism
            .ram
            .keys()
            .map(|&b| b.value())
            .filter(|&b| b < (1u64 << 32))
            .max()
            .map(|m| BFieldElement::new(m + 1))
            .unwrap_or(BFieldElement::ZERO);