[#753] Fix/drop/comment noncanonical Show instances

Description

Problem: there are many Show instances in morley that do not produce valid Haskell representations of shown terms. In particular, missing outward parens and use of Show for pretty-printing are common.

Solution: move pretty-printing into Buildable instances; make Show instances stockderived where possible; drop unnecessary Shows; reimplement necessary Shows where stock instances are impossible or unsatisfactory; comment noncanonical Shows deemed not worth the trouble with motivation for their existence and noncanonicity.

Related issue(s)

Resolves #753 (closed)

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