[WIP] Initial proposal for CancellableOneShot

[chrysn]

Add a trait that extends OneShot and allows cancellation, along with an incomplete example.

This is a first shot at #123; not ready for review as in "ensure we can merge this", but pushed into a PR to have actual discussion material in #123.

[therealprof]

Looks good to me.

[therealprof]

... but does not compile:

122 | pub trait CancellableOneShot<ADC, Word, Pin: Channel<ADC>>: OneShot<ADC, Word, Pin, Channel<ADC>> {
    |                                                                                     ^^^^^^^^^^^^ unexpected type argument

[chrysn]

Oups, tested without the unproven feature.

Fixed, and cargo +nightly test --features unproven now runs through the test as well after having ironed out the details.