moved method dispatcher implementation from OptimizeBuiltinCalls class into new MethodDispatcherTransform class in Visitor.py to enable reuse

rewrite of sliced char* decoding as utility functions, implemented efficient sliced decoding for C++ std::string

--HG--
extra : rebase_source : e134f5595be98eb990ab2195e8208940efb171fe

Fix "missing cimport: ... cython" warning.

moved C++ exception conversion utility code into new utility code file Cython/Utility/CppSupport.cpp