Support for converting into multiple int types besides repr?

[tyrone-wu]

Hi 👋.

Would you be open to adding or accepting a PR that would implement Into for multiple int types?

For context, the codebase I'm working with has several generated enums. We're currently pushing to eliminate as much as castings and funnel all enum -> int conversions through a less "foot-gunny" API. In some instances, we're using #[repr(u32)] enums as a u64.

What's currently barring us from using https://github.com/rust-num/num-derive is it assumes the variant can validly be represented as a i64. It doesn't take into consideration the repr, which would defeat the purpose of eliminating this footgun. Even though it'll be unlikely we'll encounter this issue, we would still like to be safe 🧷.