[python] Enable math.factorial (#141)

python/port/genhdr/qstrdefs.in.h

@@ -181,6 +181,7 @@ Q(exp)
 Q(expm1)
 Q(extend)
 Q(fabs)
+Q(factorial)
 Q(filter)
 Q(find)
 Q(float)

python/port/mpconfigport.h

@@ -82,6 +82,12 @@
 // Whether to provide special math functions: math.{erf,erfc,gamma,lgamma}
 #define MICROPY_PY_MATH_SPECIAL_FUNCTIONS (1)
 
+// Whether to provide math.factorial function
+#define MICROPY_PY_MATH_FACTORIAL (1)
+
+// Whether math.factorial is large, fast and recursive (1) or small and slow (0).
+#define MICROPY_OPT_MATH_FACTORIAL (0)
+
 // Whether to provide "cmath" module
 #define MICROPY_PY_CMATH (1)