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>
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