Crypto: Remove repeated code in Merkle tree backends

[schouhy]

Remove repeated code in Merkle tree backends

Description

This PR leverages const generics to remove repeated code in crypto/src/merkle_tree/backends. We previously had two separate implementations for 256 and 512 bit in both batched and single element backends. This PR merges both into a single implementation for batch and a single implementation for single elements.

Type of change

• Refactor

[codecov-commenter]

Codecov Report

Merging #584 (ce292fa) into main (c718c5d) will increase coverage by 0.00%.
The diff coverage is 98.97%.

@@           Coverage Diff           @@
##             main     #584   +/-   ##
=======================================
  Coverage   95.49%   95.49%           
=======================================
  Files         112      110    -2     
  Lines       19008    18966   -42     
=======================================
- Hits        18151    18111   -40     
+ Misses        857      855    -2     

Files	Coverage Δ
crypto/src/merkle_tree/backends/batch.rs	99.03% <100.00%> (ø)
crypto/src/merkle_tree/merkle.rs	100.00% <100.00%> (ø)
crypto/src/merkle_tree/backends/single.rs	97.82% <97.82%> (ø)

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[MauroToscano]

Maybe we can find a better name for singlebackend ?

[schouhy]

its indeed an ugly name. Other options I considered are:

1. FieldElementBackend instead of SingleBackend and FieldElementVectorBackend instead of BatchBackend, referring to the data they work with.
2. Just Backend instead of SingleBackend
3. Just Backend for both of them and use them as batch::Backend and single::Backend

[MauroToscano]

I think 1 Is nice. Also FEVector is more descriptive than Batch. We can change that change later though, I don't mind