Add key-value support

[bradjc]

This adds the api and an example app for key-value.

This is blocked on two things:

1. platform: subscribe: add return code variants #487 - we need the new upcall conversion types
2. KV: Separate virtualization from KV implementation, New KV trait, Set/Add/Update tock#3551 - we should wait until the kernel settles down before actually merging this. The userspace code won't change that much, and I wanted to open this PR to get any feedback.

[jrvanwhy]

Are you planning to add a fake version of the key-value syscall driver once tock/tock#3551 is stabilized? If so, great. If not, then we should probably have a discussion on whether we want to require unit tests for all API implementations.

[bradjc]

I added a unittest implementation.

[bradjc]

This should be ready to go.

[jrvanwhy]

This reformatting looks accidental.