All other tablegen files in this directory have the license header, but
`Common.td` is missing it

This adds a new device info query for the maximum workgroup/block size
for each dimension.

- Update the main README to reflect the current project status
- Rework the main API generation documentation. General fixes/tidying,
but also spell out explicitly how to make API changes at the top of the
document since this is what most people will care about.

---------

Co-authored-by: Martin Grant <martingrant@outlook.com>

This makes several small changes to how the platform and device info
queries are handled:
* ReturnHelper has been replaced with InfoWriter which is more explicit
  in how it is invoked.
* InfoWriter consumes `llvm::Expected` rather than values directly, and
  will early exit if it returns an error.
* As a result of the above, `GetInfoString` now correctly returns errors
  rather than empty strings.
* The host device now has its own dedicated "getInfo" function rather
  than being checked in multiple places.

`olShutDown` was not properly calling deinit on the platforms, resulting
in random segfaults on AMD devices.

As part of this, `olInit` and `olShutDown` now alloc and free the
offload context rather than it being static. This
allows `olShutDown` to be called within a destructor of a static object
(like the tests do) without having to worry about destructor ordering.

Rather than creating a new device info tree for each call to
`olGetDeviceInfo`, we instead do it on device initialisation. As well
as improving performance, this fixes a few lifetime issues with returned
strings.

This does unfortunately mean that device information is immutable,
but hopefully that shouldn't be a problem for any queries we want to
implement.

This also meant allowing offload initialization to fail, which it can
now do.

This allows removal of a specific Image from a Device, rather than
requiring all image data to outlive the device they were created for.

This is required for `ol_program_handle_t`s, which now specify the
lifetime of the buffer used to create the program.

Previously, if a binary failed to load due to failures when jit
compiling, the function would return success with nullptr. Now it
returns a new plugin error, `COMPILE_FAILURE`.

Rather than being "stringly typed", store values as a std::variant that
can hold various types. This means that liboffload doesn't have to do
any string parsing for integer/bool device info keys.

All entry points (except olInit) now check that offload has been
initialized. If not, a new `OL_ERRC_UNINITIALIZED` error is returned.