[#897] Add Constrained utility existential

NB: Some awkward instances could be simplified with GEq and GCompare, but that's left for #898 (closed).

Description

Problem: We use constrained existentials over GADTs in at least two places:

• Morley.Tezos.Address.ConstrainedAddress
• Morley.Michelson.Typed.Existential.SomeConstrainedValue

These two are very similar. Makes sense to unify them.

Solution: Introduce Constrained existential, similar to Some from some, but with additional constraints captured.

Related issue(s)

Resolves #897 (closed)

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