Performance optimization for internal methods

Internal methods like parse_statement, execute_text et. al. use METH_VARARGS in the PyMemberDef structure and parse the arguments via PyArg_ParseTuple.

Since we don't need to check the types for variable arguments we should use METH_FASTCALL (which is available since Python 3.7) and declare the functions as _PyFuncFastCall.

Methods which only expect a single parameter should use METH_O.