[enhancement] variant_count!<> builtin

### What's hard to do? (limit 100 words)

Similar to `u32::MAX`, it would be nice to have a way to get the number of elements in an enum. Similar to rust's [std::mem::variant_count](https://doc.rust-lang.org/std/mem/fn.variant_count.html).

### Current best alternative workaround (limit 100 words)

Make a const yourself and hope you keep it up to date.

### Your view of the "best case XLS enhancement" (limit 100 words)

A builtin `std::variant_count!<>` or a colon-ref (like `u32::MAX`), e.g. EnumType::NUM_VARIANTS.