[#757] Classified instructions (GADT version)

I spent more on this than was probably necessary, however the result is somewhat not terrible. How useful is it beyond Morley.Michelson.Typed.Util is debatable, however.

Further work (like #807) can perhaps expand on this.

Description

Problem: we often want to match on a typed instruction that has a certain property. To do that, we either have to match on all 'Instr' constructors, which is an ungodly amount of boilerplate, or use wildcard matches and thus introduce maintainability issues.

Solution: Introduce GADT-based machinery for classifying 'Instr' constructors in a few different ways and matching against those. Defining new classifications isn't much fun, but this seems better than doing the corresponding pattern-matches ad-hoc each time. Apply this machinery where appropriate, where it's not enough use TH (sparingly).

Related issue(s)

Resolves #757 (closed)

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