Re-order ops.

This is mostly aimed at bringing related things together, where they
ended up scattered because it's typically easier to add things at the
end. Also add a note that from now on, this order should be considered
API, with new things to be added at the end.

The order is operationally unimportant; putting related things near to
each other is simply a convenience for those working on the VM.
Consistently use 32-bit string indexes.

No need to make the same mistakes as the JVM, especially when (a) we
hit the limit and had to do workarounds when doing Rakudo JVM, and
(b) "it was the 90s!" ain't a valid excuse for us.