Create a wrapper for Word

[vlopes11]

Currently Word is a type alias

crypto/src/lib.rs

Lines 24 to 34 in 0c242d2

	// TYPE ALIASES
	// ================================================================================================
	/// A group of four field elements in the Miden base field.
	pub type Word = [Felt; WORD_SIZE];
	// CONSTANTS
	// ================================================================================================
	/// Number of field elements in a word.
	pub const WORD_SIZE: usize = 4;

However, it is a critical component of our stack as it is used everywhere, in different forms. One example is RpoDigest:

crypto/src/hash/rpo/digest.rs

Lines 7 to 11 in 0c242d2

	// DIGEST TRAIT IMPLEMENTATIONS
	// ================================================================================================
	#[derive(Debug, Default, Copy, Clone, Eq, PartialEq)]
	pub struct RpoDigest([Felt; DIGEST_SIZE]);

They both have the same structure, but different concrete types. This leads to many cases where unsafe optimizations are required, such as in here:

#58

One of the problems highlighted by this PR is the need to convert between Word and RpoDigest efficiently (i.e. without copying/allocating) & safely.

We could achieve major simplification if:

• Word was a struct
• The digest of Rpo256 was a Word

[vlopes11]

@bobbinth should we include this on 0.6? It will clean the code quite a bit as we won't have to convert between the types anymore

[bobbinth]

Yes - let's do it!