kernel/vm_manager: Handle shrinking of the heap size within SetHeapSize()

One behavior that we weren't handling properly in our heap allocation
process was the ability for the heap to be shrunk down in size if a
larger size was previously requested.

This adds the basic behavior to do so and also gets rid of HeapFree, as
it's no longer necessary now that we have allocations and deallocations
going through the same API function.

While we're at it, fully document the behavior that this function
performs.