A simple info query for events that returns whether the event is
complete or not.

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);
```

This more closely matches the nomenclature used by CUDA, AMDGPU and
the plugin interface.

Add info queries for queues and events.

`olGetQueueInfo` only supports getting the associated device. We were
already tracking this so we can implement this for free. We will likely
add other queries to it in the future (whether the queue is empty, what
flags it was created with, etc)

`olGetEventInfo` only supports getting the associated queue. This is
another thing we were already storing in the handle. We'll be able to
add other queries in future (the event type, status, etc)

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.