Split degenerate BlockSupportsPeras instance #247

We need to move away from the existing global BlockSupportsPeras blk instance to allow for real votes and certificates to be used in Cardano, while still being able to use mocked types for testing.

The ultimate goal of this ticket is to have multiple BlockSupportsPeras instance for the different block types in the codebase.

A side-effect of this change is the need to adapt call sites that previously relied on forall blk. BlockSupportsPeras blk to instead use the correct constraints needed depending on the context.