Fix pedersen padding

[rozbb]

Description

This fixes the Pedersen padding bug #76. To do this, I simply pad the input bits rather than bytes, so that the padding length check succeeds. I've also changed the tests slightly so that they're regression tests. I wasn't sure the best way to do it so let me know if you'd rather the parameters be different. To demonstrate the bug, you just need NUM_WINDOWS * WINDOW_SIZE to not be divisible by 8.

closes: #76

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• [X] Targeted PR against correct branch (main)
• [X] Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• [X] Wrote unit tests
• [X] Updated relevant documentation in the code
• [X] Added a relevant changelog entry to the Pending section in CHANGELOG.md
• [X] Re-reviewed Files changed in the Github PR explorer

[rozbb]

That is already checked in the assert at the top of the function. Should this be in addition or in the place of?

[Pratyush]

Oops, good point.