Be able to convert from integers to C-like enums

[kevina]

Now that we can specify explicit discriminant numbers (see #1393) it would be nice to be able to convert from integers to the C-like enums. Doing so safely will likely require some sort of run-time check.

[kevina]

Can the typestate system be used in this context?

[fzzzy]

+1

[graydon]

I wouldn't try to wire this in to typestate yet. It does not yet support literals and, even when it did, this would be an extension above and beyond. I'd just make it a dynamic failure if you try to cast and the range is wrong, at least for now. It's an easy sort of lint to prohibit.

[catamorphism]

Dup of #2132