Rather than having every "enqueue"-type function have an output pointer
specifically for an output event, just provide an `olCreateEvent`
entrypoint which pushes an event to the queue.
For example, replace:
```cpp
olMemcpy(Queue, ..., EventOut);
```
with
```cpp
olMemcpy(Queue, ...);
olCreateEvent(Queue, EventOut);
```
Adds a `check-offload-unit` target for running the liboffload unit test
suite. This unit test binary runs the tests for every available device.
This can optionally filtered to devices from a single platform, but the
check target runs on everything.
The target is not part of `check-offload` and does not get propagated to
the top level build. I'm not sure if either of these things are
desirable, but I'm happy to look into it if we want.
Also remove the `offload/unittests/Plugins` test as it's dead code and
doesn't build.
Implement the complete initial version of the Offload API, to the extent
that is usable for simple offloading programs. Tested with a basic SYCL
program.
As far as possible, these are simple wrappers over existing
functionality in the plugins.
* Allocating and freeing memory (host, device, shared).
* Creating a program
* Creating a queue (wrapper over asynchronous stream resource)
* Enqueuing memcpy operations
* Enqueuing kernel executions
* Waiting on (optional) output events from the enqueue operations
* Waiting on a queue to finish
Objects created with the API have reference counting semantics to handle
their lifetime. They are created with an initial reference count of 1,
which can be incremented and decremented with retain and release
functions. They are freed when their reference count reaches 0. Platform
and device objects are not reference counted, as they are expected to
persist as long as the library is in use, and it's not meaningful for
users to create or destroy them.
Tests have been added to `offload.unittests`, including device code for
testing program and kernel related functionality.
The API should still be considered unstable and it's very likely we will
need to change the existing entry points.
