Included assert.h in Python.h -- it's absurd that this basic tool of

Included assert.h in Python.h -- it's absurd that this basic tool of
good C practice hasn't been available to everything all along.
Added Py_SAFE_DOWNCAST(VALUE, WIDE, NARROW) macro to pyport.h; this
just casts VALUE from type WIDE to type NARROW, but assert-fails if
Py_DEBUG is defined and info is lost due to casting.
Replaced a line in Fredrik's fix to marshal.c to use the new macro.