Shared I2C in RTIC - exercise

Basically, this solution consists of 2 major elements.

I2cHandlerProxy which pretends to be the embedded_hal's i2c implementation. It is supposed to be consumed by a driver object during init phase.
- It spawns i2c-related task via RTIC's Spawn object.
ScopedTaskSpawnProvider which creates something similar to scope.
- On creation, it populates I2cHandlerProxy with Spawn instance.
- On drop, it sets I2cHandlerProxy Spawn reference to None.
  - After drop, accessing i2c via I2cHandlerProxy causes panic.

I reworked it and now it is runnable in qemu-system-arm with cargo run.

Major soundness issues

STSP accepts any Spawn object (actually any object that implements I2cHandlerCallable)
- Therefore, it will be transmuted into any other I2cHandlerCallable implementing type causing UB.
- It is a result of struggles with lifetimes. More information in the code and comments.
I2cCommand holds references with static lifetimes. Passing it to any spawned task with lower priority than i2c handler is a potential use-after-free.