Use abstract critical sections for thumbv6

[Sizurka]

This changes the critical sections required for thumbv6 to use the abstraction provided by the critical_section crate. The context here is the RP2040 micocontroller, which is a two core Cortex-M0+ device, so just disabling interrupts on the current core is not sufficient when a bbqueue is used to communicate between cores. However, the HAL provides an implementation of a critical section hooking into the abstraction above to handle this kind of case.

With this change, using bbqueue on single core thumbv6 target (equivalent to the current behavior) looks like:

[dependencies]
bbqueue = { version = "0.5", features = ["thumbv6"] }
cortex-m =  { version = "0.7.6", features = ["critical-section-single-core"] }

Or for a RP2040 in multi-core mode:

[dependencies]
bbqueue = { version = "0.5", features = ["thumbv6"] }
rp2040-hal = { version = "0.8.0", features=["critical-section-impl"] }

[ithinuel]

The change seems good to me. This change makes it portable to any target.

@jamesmunns is it ok to do breaking change before next release?

If so, I think the thumbv6 feature could be removed in favor of the default critical-section feature introduced by the optional flag since it is no longer thumbv6m specific.

[jamesmunns]

@ithinuel please feel free to make any breaking changes! I'm open to either using the critical-section crate, or moving to portable-atomic, which can use critical-section as necessary for targets which don't support atomics natively.

[ithinuel]

Migration to portable-atomic (plus a few other tiny changes) in #103

[Sympatron]

Closing in favor of #103.