[#847] Refactor typechecker monad

Description

Problem: 'TypeCheck' &c monads are a) using 'State' where a 'Reader' would be enough and b) are using multiple awkward lifts to handle nested reader stacks.

Solution: Implement 'MultiReaderT', a poor man's extensible reader effects via nested 'ReaderT' using recursive closed type families and Peano arithmetic. Replace 'State' with 'ReaderT'; reorder the stack to put all readers on top; use 'MultiReaderT' machinery to simplify accessing reader envs.

Other options considered and rejected:

• Use vinyl records as the reader environment. This would work, but fiddling with subtyping constraints with the simplified subsumption seemed like a lot of menial work.
• Use extensible records via a type parameter. This would work and require less type family magic, but at the cost of record field access being more awkward.

Related issue(s)

Resolves #847 (closed)

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