Require Applicative parse when creating encoders

[alexfmpe]

#984 required me to state a law in terms of Applicative parse within a function that doesn't actually have that constraint. It turns out EncoderImpl itself has that awkwardness. One way of addressing that is to require Applicative (and thus pure) to exist when constructing such values.

I have:

• Based work on latest develop branch
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• Run the test suite: $(nix-build -A selftest --no-out-link)
• Updated the changelog
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