Add serialize traits to LabeledCommitment

[TakodaS]

Description

Added CanonicalSerialize/Deserialize traits to the LabeledCommitment struct.

When poly-commit was updated to v0.4.0, the trait ToBytes was removed from LabeledCommit but the Serialization traits were not added.

Blocks arkworks-rs/marlin/pull/94 as LabeledCommit structs need to be absorbed by the sponge function to add entropy.

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

[Pratyush]

I think we shouldn't do this, as the labels exist purely at runtime; the versions absorbed into the sponge should be the unlabelled commitments.

[TakodaS]

I think we shouldn't do this, as the labels exist purely at runtime; the versions absorbed into the sponge should be the unlabelled commitments.

Having delved further into the marlin repo, I agree. Closing.