Replace `RefIs` with `RefIsNull` (#5401)

* Replace `RefIs` with `RefIsNull`

The other `ref.is*` instructions are deprecated and expressible in terms of
`ref.test`. Update binary and text parsing to parse those instructions as
`RefTest` expressions. Also update the printing and emitting of `RefTest`
expressions to emit the legacy instructions for now to minimize test changes and
make this a mostly non-functional change. Since `ref.is_null` is the only
`RefIs` instruction left, remove the `RefIsOp` field and rename the expression
class to `RefIsNull`.

The few test changes are due to the fact that `ref.is*` instructions are now
subject to `ref.test` validation, and in particular it is no longer valid to
perform a `ref.is_func` on a value outside of the `func` type hierarchy.

0 files changed, 0 insertions, 0 deletions