Enables one of two alternate memory
allocators. Only one can be enabled at a time. The alternate
allocators are used when SQLite is in “heap” mode. This allows
the application to provide a static chunk of memory that SQLite
will use for all of its internal allocations. This is most
commonly done with embedded systems where memory usage must be
carefully controlled. MEMSYS3
uses a hybrid allocation algorithm based off dlmalloc().