refactor: simplify bound method representation

[farbodahm]

Closes #389

With the help of @flub and @obestwalter, we were able to find an easy way to simplify the output of assertion failures to something like this, while keeping the old behavior same as before for unbounded objects:

=================================== FAILURES =================================== _____________________________________ test _____________________________________ def test(): > assert Help().fun() == 2 E assert 1 == 2 E + where 1 = fun() E + where fun = <test_local.Help object at 0x1074be230>.fun E + where <test_local.Help object at 0x1074be230> = Help() test_local.py:13: AssertionError =========================== 1 failed in 0.03 seconds =========================== 

However, I opened an issue (#12499) to refactor the data structure of assertions AST (in rewerite.py) from string to something more flexible.