PY-12877 Add test for the case when no expression given in 'with' header
PyWithStatementImpl implements PyStatementListContainer
Generify PyFixer class as suggested during IC-CR-417 to ged rid of casts
Move getFirstChildOfType to PyUtil. Use this method in all PyFixer descendants
Make PyStatementPart interface extend newer PyStatementListContainer

Both interfaces provide the same method getStatementList
[log] Remove VcsLogJoiner limitation: it is fast enough now
[log] Joiner: throw separate exception class if not enough data

To collect information how often such situation happens.