Allow sharing query parameters at route branches

[alexfmpe]

Edit: Rebased on top of #1079

Allows turning

data ParentRoute :: * -> * where
  ParentRoute_ChildA :: ParentRoute ()
  ParentRoute_ChildB :: ParentRoute ()
  ParentRoute_ChildC :: ParentRoute ()

  FrontendRoute_Parent -> PathSegment "parent" $ pathComponentEncoder $ \case
    ParentRoute_ChildA -> PathEnd $ unitEncoderLenient mempty
    ParentRoute_ChildB -> PathSegment "b" $ pathOnlyEncoderIgnoringQuery . unitEncoder mempty
    ParentRoute_ChildC -> PathSegment "c" $ pathOnlyEncoderIgnoringQuery . unitEncoder mempty

into

  FrontendRoute_Parent -> PathSegment "parent" $ pathComponentEncoderIgnoringQuery $ \case
    ParentRoute_ChildA -> PathEnd id
    ParentRoute_ChildB -> PathSegment "b" $ unitEncoder mempty
    ParentRoute_ChildC -> PathSegment "c" $ unitEncoder mempty

and

  FrontendRoute_Parent :: FrontendRoute (R ParentRoute)

data ParentRoute :: * -> * where
  ParentRoute_ChildA :: ParentRoute (Map Text (Maybe Text))
  ParentRoute_ChildB :: ParentRoute (Map Text (Maybe Text))
  ParentRoute_ChildC :: ParentRoute (Map Text (Maybe Text))

  FrontendRoute_Parent -> PathSegment "parent" $ pathComponentEncoder $ \case
    ParentRoute_ChildA -> PathEnd id
    ParentRoute_ChildB -> PathSegment "b" queryOnlyEncoder
    ParentRoute_ChildC -> PathSegment "c" queryOnlyEncoder

into

  FrontendRoute_Parent :: FrontendRoute (R ParentRoute, Map Text (Maybe Text))

  FrontendRoute_Parent -> PathSegment "parent" $ pathComponentEncoderSharingQuery id $ \case
    ParentRoute_ChildA -> PathEnd id
    ParentRoute_ChildB -> PathSegment "b" $ unitEncoder mempty
    ParentRoute_ChildC -> PathSegment "c" $ unitEncoder mempty

Motivation

• for the former comes from a project frequently linked to from social media, which has a habit of injecting tracking parameters and breaking all the links, which forces fooIgnoringQuery to be added all over the leaf routes
• for the later comes from a project where several sub-routes shared the same query parameters which made it necessary to add the same encoder (yielding something more structured than `queryOnlyEncoder) to all the children

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)

[alexfmpe]

Probably should be exported - I end up inlining this in every project

[alexfmpe]

Rebased on top of #1079