bus/spi: add RefCell, CriticalSection and Mutex shared bus implementations.

[Dirbaio]

Requires #443

This adds a few bus sharing implementations, with varying tradeoffs:

• RefCellDevice: single thread only
• CriticalSectionDevice: thread-safe, coarse locking, nostd.
• MutexDevice: thread-safe, fine-grained locking, std only.

[eldruin]

I hit the button too quickly in #443. Could you add entries to the changelog for the changes in #443 here or add a new PR?

[Dirbaio]

@eldruin done

[eldruin]

Thank you!
bors r+

[bors]

Build succeeded:

• clippy
• fmt
• test (1.59.0, thumbv6m-none-eabi)
• test (1.59.0, thumbv7m-none-eabi)
• test (1.59.0, x86_64-unknown-linux-gnu)
• test (nightly, thumbv6m-none-eabi)
• test (nightly, thumbv7m-none-eabi)
• test (nightly, x86_64-unknown-linux-gnu)
• test (stable, thumbv6m-none-eabi)
• test (stable, thumbv7m-none-eabi)
• test (stable, x86_64-unknown-linux-gnu)