Add postprocessor functional object to the replace_fn template. Add unit-test that demonstrates how to build a replacer that builds a trace. The trace associates new expressions with the old ones that were used to create it.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add metavariable utilities. They will be used to refactor the elaborator.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>