Add impl TryFrom<T> for E where E is a C-like #[repr(T)] enum

This is a problem that has come up a lot. The most recent discussion I could find about it is the internals thread here, but that thread has been locked and I figured this should probably have an "official" issue somewhere.

There's currently no (good) way to convert from an enum integer discriminant value to the enum itself. Enums should support deriving TryFrom for this purpose. eg.

#[repr(u32)]
#[derive(TryFrom)]
enum MyCoolEnum {
    Foo = 1,
    Bar = 2,
}

let x: u32 = ... ;
let my_cool_enum = MyCoolEnum::try_from(x).expect("invalid discriminant!");

Start by reading the linked internals thread and the requested #[derive(TryFrom)] example in this issue. Define the RFC-level behavior for C-like #[repr(T)] enums, including how invalid discriminant values are handled, and document the proposed design for review.