bus/i2c: add RefCell, CriticalSection and Mutex shared bus implementations.

[Dirbaio]

Requires #440

Same as #443 but for I2C.

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]

It looks generally good to me, thank you!
Could you add some more information about the std feature to the lib.rs?

[Dirbaio]

@eldruin done

[eldruin]

Looks good to me, 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)