Merge #110

110: Add critical-section 1.0 implementation, fix multicore unsoundness. r=almindor a=Dirbaio

~~Requires #109~~

This adds a [critical-section](https://github.com/rust-embedded/critical-section) implementation for single-core chips, based on disabling all interrupts.

`interrupt::free` is is unsound on multicore systems because it only disables interrupts in the
current core. For multicore chips, a chip-specific critical section implementationis needed instead. Unsoundness is fixed by not returning the `CriticalSection` token.

This is a breaking change.

This is the riscv equivalent of https://github.com/rust-embedded/cortex-m/pull/447 and https://github.com/rust-embedded/cortex-m/pull/448





Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>