[Pre-RFC] Make some feature-detected function-to-fn-pointer casts safe through ZST token types

Doing a function pointer cast seems to me like a very direct way to do multi-version dispatch.

Would it be any easier to implement if the cast didn't use ZST's but was just a macro? There's already is_x86_feature_detected so the main thing would be checking the target_feature on the passed method (which I suppose probably isn't known until after macro expansion but may that's solvable?)

#[target_feature(enable = "sse4.1")]
fn crazy() {
    …
}

fn boring_scalar_implementation() {
    …
}

fn choose_implementation() -> ... {
    let sse_impl: Option<_> = try_target_feature_cast!("sse4.1", crazy);

    see_impl.unwrap_or(boring_scalar_implementation)
}