support reexporting pio_asm!

[Dirbaio]

I'd like to reexport pio, pio-proc from embassy-rp, so the user doesn't have to depend on them directly. It's nicer because it ensures the right version is used, so it avoids version mismatch errors of doom.

However, currently pio_asm! generates code that does ::pio::whatever. This doesn't work unless pio is in scope in the end user's crate. I'm not sure if this is fixable with proc macros though? With decl macros you'd use $crate, but that doesn't seem to exist for proc macros.

[jannic]

As it is a function-like macro, this workaround (taken from https://internals.rust-lang.org/t/rfc-crate-for-proc-macro/16968) could work:

#[macro_export]
macro_rules! functionlike {
    ( $($tt:tt)* ) => {
        $crate::__proc_macros::functionlike! {
            #![crate = $crate]
            $($tt)*
        }
    }
}

Ie., define a macro_rules macro which can use $crate, and forward the expanded value of $crate to the proc macro explicitly.

https://github.com/Nullus157/stylish-rs/blob/stylish-macros-0.1.2/core/src/lib.rs#L121 is an example where this method is used.

[Dirbaio]

ooh that's a clever trick. I'm going to give it a try, first step is #71