test: instances of Arbitrary for MastNodeForest & BasicBlockNode

[huitseeker]

On top of #2228

• This creates instances of Arbitrary for BasicBlockNode, reusing and extending the strategies introduced in feat: Add Noops to OpBatches #2094.
• Incidentally, this uses the above to add a few serde roundtrips, reusing the macros introduced in feat: macro-generated serialization tests #2132.
• This creates a benchmark leveraging the above to generate a handful of random MastForests, and benches their merge.

The goal of this PR is to then power refactors of the mast::merger module (one of the later writers into the MastForest nodes in their life cycle).

[plafer]

LGTM

[bobbinth]

Looks good! Thank you!