py/obj: Make mp_obj_get_complex_maybe call mp_obj_get_float_maybe first.

This commit simplifies mp_obj_get_complex_maybe() by first calling
mp_obj_get_float_maybe() to handle the cases corresponding to floats.
Only if that fails does it attempt to extra a full complex number.

This reduces code size and also means that mp_obj_get_complex_maybe() now
supports user-defined classes defining __float__; in particular this allows
user-defined classes to be used as arguments to cmath-module function.

Furthermore, complex_make_new() can now be simplified to directly call
mp_obj_get_complex(), instead of mp_obj_get_complex_maybe() followed by
mp_obj_get_float().  This also improves error messages from complex with
an invalid argument, it now raises "can't convert <type> to complex" rather
than "can't convert <type> to float".

Signed-off-by: Damien George <damien@micropython.org>

tests/float/complex_dunder.py

@@ -1,6 +1,11 @@
 # test __complex__ function support
 
 
+class TestFloat:
+    def __float__(self):
+        return 1.0
+
+
 class TestComplex:
     def __complex__(self):
         return 1j + 10
@@ -20,6 +25,7 @@ class Test:
     pass
 
 
+print(complex(TestFloat()))
 print(complex(TestComplex()))
 
 try: