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>

`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.

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`.

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

This is a three element x, y, z size_t vector that can be used any place
where a 3D vector is required. This ensures that all vectors across
liboffload are the same and don't require any resizing/reordering
dances.