Extract 'dsumEncoder' from chainEncoder

[alexfmpe]

I wanted to call it dependentEncoder since it exhibits a bit of context dependent parsing, but then there's the dependent sum aspect and the name would get confusing. While a dependent sum encoder must be ctx dependent, the converse is not true. We can get a simpler

  => Encoder check parse a a'
  -> (a -> Encoder Identity parse b b')
  -> Encoder check parse (a,b) (a', b')

by wrapping/unwraping via Const. I'm tempted to add such a util for convenience but am not coming up with a good name. Maybe contextDependentEncoder on account of it being the simpler such encoder?

I have:

• Based work on latest develop branch
• Followed the contribution guide
• Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• Run the test suite: $(nix-build -A selftest --no-out-link)
• Updated the changelog
• (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[Ericson2314]

Looks good to me!

[ryantrinkle]

This looks great; thanks!

[alexfmpe]

Should I bundle a contextDependentEncoder with this?