Add unstable `core::mem::variant_count` intrinsic
Adds a new `const fn` intrinsic which can be used to determine the number of variants in an `enum`.
I've shown this to a couple of people and they invariably ask 'why on earth?', but there's actually a very neat use case:
At the moment, if you want to create an opaque array type that's indexed by an `enum` with one element for each variant, you either have to hard-code the number of variants, add a `LENGTH` variant or use a `Vec`, none of which are suitable in general (number of variants could change; pattern matching `LENGTH` becomes frustrating; might not have `alloc`). By including this intrinsic, it becomes possible to write the following:
```rust
#[derive(Copy, Clone)]
enum OpaqueIndex {
A = 0,
B,
C,
}
struct OpaqueVec<T>(Box<[T; std::mem::num_variants::<OpaqueIndex>()]>);
impl<T> std::ops::Index<OpaqueIndex> for OpaqueVec<T> {
type Output = T;
fn index(&self, idx: OpaqueIndex) -> &Self::Output {
&self.0[idx as usize]
}
}
```
(We even have a use cases for this in `rustc` and I plan to use it to re-implement the lang-items table.)