The function Expand_Hp asks the system to expand the amount of memory available
for the heap.  The Russell run-time system is normally stingy in its memory
allocation.  It starts out with no heap, and expands only after "unsuccessful"
garbage collections.  Any process which is known to use large amounts of memory
can be sped up with an initial call to Expand_Hp.  Its signature is:

        func [ val Short ] val Void

The argument is the number of additional blocks to allocate.  The size of a
block is currently defined to be 4K.