This PR:

• reexports the pio_asm and pio_file crates from the pio crate.
• Wraps them with a decl macro that uses $crate so that they don't break when reexported.

This has two benefits:

• Users can now depend only on pio instead of pio+pio_proc.
• HALs can reexport pio so the user doesn't even need to depend on pio directly, and they get the right version automatically. This has been a source of pain in embassy-rp when using git versions.

This commit shows the changes needed for HAL and user code. Works nicely. embassy-rs/embassy@65f00c5

Unfortunately to do this I've had to move the data structures to a new pio-core crate. Otherwise
pio-proc and pio would have a circular dependency.

fixes #70
depends on #71