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.