ISLE: allow declaring internal constructors as infallible

In https://github.com/bytecodealliance/wasmtime/pull/3545, we added this internal constructor:

```lisp
(decl vector_size (Type) VectorSize)
(rule (vector_size (multi_lane 8 16)) (VectorSize.Size8x16))
(rule (vector_size (multi_lane 16 8)) (VectorSize.Size16x8))
(rule (vector_size (multi_lane 32 4)) (VectorSize.Size32x4))
(rule (vector_size (multi_lane 64 2)) (VectorSize.Size64x2))
```

it generates this code:

```rust
// Generated as internal constructor for term vector_size.
pub fn constructor_vector_size(ctx: &mut C, arg0: Type) -> Option {
let pattern0_0 = arg0;
if let Some((pattern1_0, pattern1_1)) = C::multi_lane(ctx, pattern0_0) {
if pattern1_0 == 8 {
if pattern1_1 == 16 {
// Rule at src/isa/aarch64/inst.isle line 952.
let expr0_0 = VectorSize::Size8x16;
return Some(expr0_0);
}
}
if pattern1_0 == 16 {
if pattern1_1 == 8 {
// Rule at src/isa/aarch64/inst.isle line 953.
let expr0_0 = VectorSize::Size16x8;
return Some(expr0_0);
}
}
if pattern1_0 == 32 {
if pattern1_1 == 4 {
// Rule at src/isa/aarch64/inst.isle line 954.
let expr0_0 = VectorSize::Size32x4;
return Some(expr0_0);
}
}
if pattern1_0 == 64 {
if pattern1_1 == 2 {
// Rule at src/isa/aarch64/inst.isle line 955.
let expr0_0 = VectorSize::Size64x2;
return Some(expr0_0);
}
}
}
return None;
}
```

It handles all cases, but neither the Rust compiler nor the ISLE compiler can really tell that. So it would be nice if we could mark it infallible in the ISLE source somehow, and then have an `unreachable!()` at the end of the generated function, instead of `return None`.

Please commence with syntax bike shedding suggestions :)