Document key bits of SimpleSmt

[vlopes11]

Currently, the key bits are documented as 63-bit

crypto/src/merkle/simple_smt/mod.rs

Lines 9 to 13 in 398af59

	/// A sparse Merkle tree with 63-bit keys and 4-element leaf values, without compaction.
	/// Manipulation and retrieval of leaves and internal nodes is provided by its internal `Store`.
	/// The root of the tree is recomputed on each new leaf update.
	#[derive(Debug, Clone, PartialEq, Eq)]
	pub struct SimpleSmt {

However, we take u64 as arguments. This difference might create a confusion downstream.

crypto/src/merkle/simple_smt/mod.rs

Lines 153 to 154 in 398af59

	/// Inserts a leaf located at the specified key, and recomputes hashes by walking up the tree
	pub fn insert_leaf(&mut self, key: u64, value: Word) -> Result<(), MerkleError> {

One option is to, instead, take a Felt as argument. But then, we also need to rework the rpo module documentation that also mentions 64-bit - and it is important we keep consistency between these, as RPO is the backend of SimpleSmt

crypto/src/hash/rpo/mod.rs

Lines 60 to 65 in 398af59

	/// The parameters used to instantiate the function are:
	/// * Field: 64-bit prime field with modulus 2^64 - 2^32 + 1.
	/// * State width: 12 field elements.
	/// * Capacity size: 4 field elements.
	/// * Number of founds: 7.
	/// * S-Box degree: 7.

Originally posted by @grjte in #64 (comment)

[bobbinth]

Closing as no longer relevant after #245.