fetch example: use servant-client-js, remove Miso.Fetch

[amesgen]

This PR removes the Servant functionality from Miso.Fetch, instead recommending users to use servant-client-core with a backend of their choice, while recommending servant-client-js.

• Currently, we need to use a patched servant-client-js, see Update for newer servant-client-core, support GHC JS backend morganthomas/servant-client-js#6.

  Depending on upstream responsiveness, we might want to fork it, or even vendor it within Miso; it is very lightweight.

• We keep the fetchJSON function for simple use cases.

• New section in the README on how to interact with HTTP APIs.

The commits can be reviewed individually.

Background on servant-client-core

The general idea of servant-client-core is as follows:

• It provides the HasClient class, defined for all servant combinators¹.

  The Fetch class from Miso.Fetch is basically a less general reimplementation of this.

• Any Servant API satisfying HasClient can be run using a backend implementing the RunClient/RunStreamingClient class.

  There are various existing frontend/browser implementations of this, of varying quality:

  • servant-client-ghcjs, only works for GHCJS, based on XHR
    • Doesn't support the GHC JS backend AFAICT
  • servant-jsaddle, based on JSaddle, using XHR
    • Depends on ghcjs-dom which is a huge dependency.
    • Has known bugs.
  • servant-client-js, based on JSaddle, using fetch
    • Somewhat outdated, doesn't supported the latest servant versions.
    • ⭐ Upstream PR here: Update for newer servant-client-core, support GHC JS backend morganthomas/servant-client-js#6
  • servant-client-fetch, using the Wasm JSFFI, using fetch
    • I haven't looked at the code closely, but an advantage in principle of using the Wasm JSFFI directly is that things can be more efficient, e.g. when passing bytes through the FFI boundary.

  For Miso.Fetch, @georgefst essentially implemented/is implementing (Further generalise the fetch API #888) another backend, based on JSaddle and fetch (as well as starting with Wasm-specific optimizations in Further generalise the fetch API #888).

Footnotes

1. Third-party adaptors usually also provide instances in a separate -client package, see e.g. servant-auth-client. ↩