Revamp specification

Revamp the proposed specification to provide convenience APIs that are
similar to CUDA's `cudaEventRecord` and `cudaStreamWaitEvent` because
this is the immediate request from our customer.

I think we do still want to add a `record_event` property, but I think
we could add that separately as part of the KHR being proposed in
KhronosGroup/SYCL-Docs#922, or as a separate oneapi extension based on
that KHR.

Author: gmlueck