This patch implements text and binary encoding/decoding support for
the stack switching proposal. It does so by adapting the previous
typed-continunations implementation. Particular changes:

* Support for new `resume` encoding.
* Added support for `resume_throw` and `switch`.
* Feature flag `typed-continuations` has been renamed to `stack-switching`.

A small unfortunate implementation detail is that the internal name
`Switch` was already taken by the `br_table` instruction, so I opted
to give the `switch` instruction the internal name `StackSwitch`.

A minor detail is that I have reordered the declarations/definitions
of the stack switching instructions such that they appear in ascending
order according to their opcode value (this is the same order that the
stack-switching explainer document present them in).

'ContNew' and 'ContBind'

This reverts commit d1866bc.

This commit is a partial solution to removing the type immediate
members from `cont.new,cont.bind,resume,resume_throw,switch`. However,
when I fully remove the type immediates then I observe a crash in
`child-ir.h` on
`visitContBind,visitResume,visitResumeThrow,visitStackSwitch`. It seems that `curr->cont->type` is sometimes not a continuation type...