[#120] expose target type in type check functions

Description

Problem: `typeCheckValue` and a couple of related functions return
`Typed.Value t` wrapped in an existentially-quantified wrapper. However
these functions also accept something like `Notes t`, from which they
derive the actual desired type of the value being type-checked. This
seems like an unneccessary complication.

Solution: drop existential wrappers and make these function get the type
via TypeApplications.

Related issue(s)

Resolves #120 (closed)

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