[utils] Add a region allocator utility library.

Add a small utility library (and tests) to manage allocating regions
of a 64-bit space. The general idea is that a user of the library can
create an allocator and add some (non-overlapping) regions to it.
They can then allocate regions from the available regions in the
allocator, either by size and alignment requirement or by specific
base address and size.

C and C++ APIs are provided. The C++ API depends on mxtl, but does
not depend on any particular implementation of new. Instead, it
explicitly uses malloc/free under the hood. Bookkeeping allocation is
done using allocation pools which allocate and carve up slabs of
memory (placing the results on a free list) in order to minimize the
frequency of trips to the heap and avoid heap fragmentation. Limits
can be placed on the total size of a pool, and pools can be shared
across multiple allocators allowing multiple applications in the same
binary to compartmentalize their memory impact.

Change-Id: I007322b66f60c2e520e45013e9c3352a60557af0