wasm: revamp JSFFI internal implementation and documentation

This patch revamps the wasm backend's JSFFI internal implementation
and documentation:

- `JSValManager` logic to allocate a key is simplified to simple
  bumping. According to experiments with all major browsers, the
  internal `Map` would overflow the heap much earlier before we really
  exhaust the 32-bit key space, so there's no point in the extra
  complexity.
- `freeJSVal` is now idempotent and safe to call more than once. This
  is achieved by attaching the `StablePtr#` to the `JSVal#` closure
  and nullifying it when calling `freeJSVal`, so the same stable
  pointer cannot be double freed.
- `mkWeakJSVal` no longer exposes the internal `Weak#` pointer and
  always creates a new `Weak#` on the fly. Otherwise by finalizing
  that `Weak#`, user could accidentally drop the `JSVal`, but
  `mkWeakJSVal` is only supposed to create a `Weak` that observes the
  `JSVal`'s liveliness without actually interfering it.
- `PromisePendingException` is no longer exported since it's never
  meant to be caught by user code; it's a severe bug if it's actually
  raised at runtime.
- Everything exported by user-facing `GHC.Wasm.Prim` now has proper
  haddock documentation.
- Note [JSVal representation for wasm] has been updated to reflect the
  new JSVal# memory layout.

(cherry picked from commit fd40eaa1)