Use critical_section::CriticalSection for GPIO configuration

[urdh]

This change improves interoperability with RTICv2 (which exposes a bare_metal::CriticalSection during initialization).

[TeXitoi]

We can’t do that as is. We need at least to provide a safe way of getting GPIOs.

Do you have any reference on another hal doing something similar, or a guideline about the removal of the token in cortex_m::interrupt::free?

[urdh]

As far as I can tell, most HALs don't require a critical section at all when reconfiguring GPIOs.

Upstream actually recommends critical_section::with as a replacement for cortex_m::interrupt::free, but that provides a different CriticalSection which is incompatible with the bare_metal one. Still, this crate should probably use critical_section rather than bare_metal.

(I guess ultimately I should log an issue with RTIC (again) to have RTIC use critical-section; in the meantime interfacing between the two will require a tiny bit of unsafe code.)