We implement the PreciseSystemState with a more efficient variant. It
uses the __slot__ feature of python (therefore I removed it from the
inheritance chain, which causes some copying of code).

With this change the memory consumption for bench-coptermock drops from
~60Mb to ~40Mb for the SymbolicSystemExecution pass. With pypy3, the
memory consumption drops to 23Mbyte.

Change-Id: Ic87be8afa97d6861dfe49c9cffd40d3bdc6464a5