handle kernel <> arrow type mappings via re-usable logic

[roeap]

It seems like we have several of these arrow/kernel type pairs. I wonder if there's a way to capture the pairings just once and reuse them? Maybe some kind of trait?

enum Foo {
    INT,
    STRING,
}
trait FooFromBar {
    const F: Foo;
}

impl FooFromBar for i32 {
    const F: Foo = Foo::INT;
}

impl FooFromBar for String {
    const F: Foo = Foo::STRING;
}

Originally posted by @scovich in #270 (comment)