Update explainer for the new spec (#137)

This updates the explainer text according to the new spec we agreed in
the 09-15-2020 CG meeting and discussions afterwards.

The following are modifications and clarifications we made after the
09-15-2020 CG meeting, and the relevant issue posts, if any:
https://github.com/WebAssembly/meetings/blob/master/main/2020/CG-09-15.md

- `catch_br` wasm renamed to `delegate` (#133)
- `rethrow` gains an immediate argument (#126)
- Removed dependences on the reference types proposal and the multivalue
  proposal. The multivalue proposal was previously listed as dependent
  because 1. `try` is basically a `block`, so it can have multivalue
  input/output 2. `br_on_exn` can extract multiple values from a
  `block`. We don't have `br_on_exn` anymore, and I'm not sure 1 is a
  strong enough reason to make it a dependence.
- Mention `rethrow` cannot rethrow exceptions caught by `unwind` (#142
  and #137)
- Mention some runtimes, especially web VMs, can attach stack traces to
  the exception object, implying stack traces are not required for all
  VMs
- Update label/validation rules for `delegate` and `rethrow` (#146)
- Finalize opcodes for `delegate` (0x18) and `catch_all` (0x19) (#145
  and #147)

I believe this resolves many previous issue threads, so I'll close them.
Please reopen them if you think there are things left for discussions in
those issues.

Resolves #113, resolves #126, resolves #127, resolves #128, resolves
#130, resolves #142, resolves #145, resolves #146, resolves #147.

README.md

@@ -8,8 +8,6 @@ for adding exception handling to WebAssembly.
 * See the [proposal overview](proposals/exception-handling/Exceptions.md) for a
 summary of the proposal.
 
-The repository is now based on the [reference types proposal](proposals/reference-types/Overview.md) and includes all respective changes.
-
 Original README from upstream repository follows...
 
 # spec