WIP: route: Simplify imlp of pathParamEncoder, pathLiteralEncoder

[3noch]

Ported from @xplat's changes elsewhere.

I have:

• [ ] Based work on latest develop branch
• [ ] 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)
• [ ] (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)