kernel: subsys: lib: drivers: Use k_heap instead of z_mem_pool wrappers

Use the core k_heap API pervasively within our tree instead of the
z_mem_pool wrapper that provided compatibility with the older mempool
implementation.

Almost all of this is straightforward swapping of one alloc/free call
for another.  In a few cases where code was holding onto an old-style
"mem_block" a local compatibility struct with a single field has been
swapped in to keep the invasiveness of the changes down.

Note that not all the relevant changes in this patch have in-tree test
coverage, though I validated that it all builds.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>