CUDA: Deprecate eager compilation of device functions - #7114

In future versions of Numba, the ``device`` kwarg to the ``@cuda.jit`` decorator
will be obviated, and whether a device function or global kernel is compiled will
be inferred from the context. With respect to kernel / device functions and lazy
/ eager compilation, four cases are presently handled:

1. ``device=True``, eager compilation with a signature provided
2. ``device=False``, eager compilation with a signature provided
3. ``device=True``, lazy compilation with no signature
4. ``device=False``, lazy compilation with no signature

The latter two cases can be differentiated without the ``device`` kwarg, because
it can be inferred from the calling context - if the call is from the host, then
a global kernel should be compiled, and if the call is from a kernel or another
device function, then a device function should be compiled.

The first two cases cannot be differentiated in the absence of the ``device``
kwarg - without it, it will not be clear from a signature alone whether a device
function or global kernel should be compiled. In order to resolve, this, support
for eager compilation of device functions will be removed. Eager compilation
with the ``@cuda.jit`` decorator will in future always imply the immediate
compilation of a global kernel.

Co-authored-by: stuartarchibald <stuartarchibald@users.noreply.github.com>

Suggested in PR numba#7114 feedback.