Introduce support for a slabbed allocator: (#4218)

When instantiating a large amount of fixed-sized objects on the heap
the overhead that dynamic memory allocation APIs impose will quickly
become significant.

In some cases, allocating a large amount of memory at once and using
a slabbing allocator to carve the large block into fixed-sized units
that are used to service requests for memory out will help to reduce
memory fragmentation significantly and, potentially, improve overall
performance.

This commit introduces a new `SlabAllocator<>` class that exposes an
API that is _similar_ to the C++ concept of an `Allocator` but it is
not meant to be a general-purpose allocator.

It should not be used unless profiling and analysis of specific memory
allocation patterns indicates that the additional complexity introduced
will improve the performance of the system overall, and subsequent
profiling proves it.

A helper class, `SlabAllocatorSet<>` simplifies handling of variably
sized objects that benefit from slab allocations.

This commit incorporates improvements suggested by Greg Popovitch
(@greg7mdp).

Commit 1 of 3 in #4218.