tightly bound 'Number'’s type

`Number` describes its type as an unbounded range:

```cpp
Ptr<TypeExpr> Number::type() const {
  return Ptr<Range>::make(nullptr, nullptr, location());
}
```

Offhand, I don’t recall why this would be. It seems much more natural for it to return a type that contains only its exact value. This would allow more precise reasoning elsewhere, especially in upcoming `union` support.