feat: expose sha256 helper functions

[saleel]

Description

Expose sha256 internal methods

Problem*

Expose these functions so they could be used for partial-sha256 implementation (by Mach34/zkemail.nr). Currently these methods are copy/pasted.

Summary*

Make below functions public:

• verify_msg_block
• build_msg_block_iter
• msg_u8_to_u32
• pad_msg_block
• attach_len_to_msg_block
• hash_final_block

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[TomAFrench]

Internal methods are internal on purpose for reasons set out in #1258

The more stuff we expose through the stdlib, the harder it is to make non breaking changes. Copy-pasting this code is fine.

[saleel]

The more stuff we expose through the stdlib, the harder it is to make non breaking changes.

True. Closing.