fix(math/bitslice): fix partition upper part range check width

[ivokub]

Description

Fixes #1153.

Also handle separately case when nbDigits >= Fr width as we need to check that we do not overflow and use unique decomposition. There isn't a better way beyond doing bit-by-by comparison against the modulus, so this case is deferred to math/bits package.

Added some package documentation.

Shouldn't affect current usage of the implementation as we call everywhere with nbDigits small.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

• TestIssue1153

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules