Feature: add support for using MLSumcheck as subprotocol

[JayWhite2357]

Description

Some protocols use sumcheck as a subprotocol. Two components in MLSumcheck that don't enable this well are:

• The rng is not linked to the broader protocol. (i.e., the transcript begins and ends within sumcheck). This is part of what Use Sponge for Transcript #54 is trying to solve.
• The prover's state is not accessible after the prover runs. This is mentioned in Return of MLSumcheck::prove #64.

This PR adds prove_as_subprotocol and verify_as_subprotocol functions to MLSumcheck. As a result, this is not a breaking change. (Although I don't see the utility of the existing verify and prove functions.)

Note: I did not use Sponge since it's on 0.3.0, and seems to be a WIP.

Finally, some tests around this are added/improved.

closes: #64

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• Targeted PR against correct branch (master)
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• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
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[Pratyush]

Thanks for the PR!