Make theory rewriters non-static (#3547)

Make theory rewriters non-static (#3547)

This commit changes theory rewriters to be non-static. This refactoring
is needed as a stepping stone to making our rewriter configurable: If we
have multiple solver objects with different rewrite configurations, we
cannot use `static` variables for the rewriter table in the BV rewriter
for example. It is also in line with our goal of getting rid of
  singletons in general. Note that the `Rewriter` class is still a
  singleton, which will be changed in a future commit.