Fix bug with Variable _MEM
Fix bug with Variable _MEM

In order to get the correct unique, global index, all variables need
to be listed in a flat memory.

pombredanne/pyeda,cjdrake/pyeda,GtTmy/pyeda,sschnug/pyeda,karissa/pyeda,karissa/pyeda,cjdrake/pyeda,sschnug/pyeda,karissa/pyeda,GtTmy/pyeda,pombredanne/pyeda,GtTmy/pyeda,pombredanne/pyeda,sschnug/pyeda,cjdrake/pyeda