boilerplate: add heapmem allocator
This is the umpteenth incarnation of the McKusick allocator, aimed at replacing other allocators which have serious issues: - TLSF is fast but not that good when it comes to memory overhead with small sizes (i.e. < 512 bytes) on 64bit. - heapobj-pshared has decent overhead figures but may be insanely slow at deallocating blocks from large, hot heaps. - the Cobalt core allocator is even worse than heapobj-pshared in deallocating blocks, although the system heap should be generally small enough not to trigger the most pathological cases in practice, hopefully. Nevertheless, the performances are unacceptable. The original McKusick algorithm implements a quick fit allocation scheme, based on bucket management of ^2-sized chunks, which automatically switches to a page-based allocation method for blocks larger than twice the base page size. This variant maintains the free page list in AVL trees for fast lookups of multi-page memory ranges, and pages holding bucketed memory have a quick allocation bitmap to manage their blocks internally. The current implementation can replace TLSF for delivering process-local memory with similar performances but lesser overhead with small sizes. Most importantly, a kernel variant of HEAPMEM should replace the Cobalt core allocator. Likewise, heapobj-pshared which is beyond repair should be replaced with a process-shareable variant as well, assuming the average size and allocation patterns of real-time objects are similar in all contexts.