This patch adds a new Python C API called PyString_AsStringAndSize()

This patch adds a new Python C API called PyString_AsStringAndSize()
which implements the automatic conversion from Unicode to a string
object using the default encoding.

The new API is then put to use to have eval() and exec accept
Unicode objects as code parameter. This closes bugs #110924
and #113890.

As side-effect, the traditional C APIs PyString_Size() and
PyString_AsString() will also accept Unicode objects as
parameters.