[#576] Avoid unnecessary contract conversions

Description

Problem: some code paths in Cleveland repeatedly convert typed contract to untyped and back.

Solution: in two parts:

• Fold the zoo of Cleveland's *OriginateData types into a single tagged GADT.
• Make Cleveland's internal functions polymorphic in the tag.

The rest kind of falls out from the above automatically with minimal effort -- we just don't convert representations midway, and only do that at the very end before handing off to morley/morley-client.

This additionally does a bit of refactoring, as switching to a tagged GADT makes some old code dealing with open types redundant.

Related issue(s)

Resolves #576 (closed)

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