Add a format specifier %R to PyUnicode_FromFormat(), which embeds

the result of a call to PyObject_Repr() into the string. This makes
it possible to simplify many repr implementations.

PyUnicode_FromFormat() uses two steps to create the final string: A first
pass through the format string determines the size of the final string and
a second pass creates the string. To avoid calling PyObject_Repr() twice
for each %R specifier, PyObject_Repr() is called during the size
calculation step and the results are stored in an array (whose size is
determined at the start by counting %R specifiers).