Bad method summaries derived for complex flow

[formanek]

Because of the direct Taint instances modifications, the Taint class had its equals method defined only for the state attribute - full equals caused an error from the issue #82. For this reason, the analysis sometimes finished too early (with less iterations than actually needed) and some parameter was missing in the summary or non-parametric taint was invalid. Although this affected less than 1% of method summaries, it could cause both false positives and false negatives. This is fixed by not modifying values in frames directly and implementing proper equals method for Taint.