ISLE should have `match` expressions

If I want to match on something in an RHS, I have to split the continuation out into a new rule:

```wat
(type A extern (enum))
(type B extern (enum I J K))
(type C extern (enum))

(decl a_to_b (A) B)
(extern constructor a_to_b my_a_to_b_impl)

(decl foo (A) C)
(rule (foo a)
(foo2 (a_to_b a)))

(decl foo2 (B) C)
(rule (foo2 (B.I)) ...)
(rule (foo2 (B.J)) ...)
(rule (foo2 (B.K)) ...)
```

I wish that I didn't need to define a new term, and could instead match on `(a_to_b a)` directly inside `foo`:

```wat
(rule (foo a)
(match (a_to_b a)
((B.I) ...)
((B.J) ...)
((B.K) ...)))
```

Some things to think about: partiality and side effects.

But since we can already write this today, just with two terms instead of one, I don't think we should have any show stoppers here.