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.