Really filter out useless stack traces.
Assertion to verify that no function is patched twice.
Pep8 fix.
Fix references to patched functions.

When a function belonging to a module is patched, its copy in other
modules are not touched. This commit add a second scan of the modules
in order to spot such copies and fix them.
Add a test for the fix references feature.
Do not fail if a function is met twice.

Do not patch it twice, either.