add SplitBytesToWords circuit to bytes.circom

[remicolin]

Description

Add SplitBytesToWords circom template to bytes.circom helper file. The introduced circuit allows to split an array of bytes into an array of words, which is useful for casting a message or modulus before RSA verification.

Type of Change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] I have discussed with the team prior to submitting this PR
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] New and existing unit tests pass locally with my changes

[remicolin]

I will add another circuit and test to split words into words with a different n and k