Unstable: `&'static str` from Encoding

[madsmtm]

Idea is that we generate an array in a const fn, and then get a reference to that array and put it in a const.

Requires https://github.com/rust-lang/rust/issues/76560.

Another approach would be to require Encode implementors to add a macro to the end, like this:

unsafe impl Encode for MyType {
    const ENCODING: Encoding<'static> = ...;
    end_encode_impl!();
}

[madsmtm]

Though of course https://github.com/rust-lang/const-eval/issues/20 would make this much, much easier!