Dedicated allocator for Wi-SUN critical features#

The Wi-SUN alloc component provides a separate heap dedicated to critical Wi-SUN features, including cryptographic operations. It redirects MbedTLS calloc and free functions to a custom allocator working with a static array of configurable size (defaults to 16kB). The purpose of this heap is to make the Wi-SUN stack more reliable under heavy heap usage by reserving enough space to always be able to maintain connectivity. In the event that the heap runs out of memory, the custom allocator defaults back to standard functions as a best effort strategy. Note: all cryptographic operations may allocate on this heap; if the application requires more operations than those needed by Wi-SUN, the heap should be dimensioned accordingly. Use sl_wisun_heap_get_stats() to help track heap usage if need be.