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>
