MAINT: remove explicit xp.complex128 imports

[ev-br]

towards #431

1. Refactor the scalars strategy.

While at it, note that complex64 cases used to generate values of 32 bits for both real and imag parts: so in a range of complex32 / two float16.

2. Refactor the float <-> complex assertions (used in FFT tests)

TODO: test locally. The scalars strategy is fairly deep in the stack. done, tested locally, no new problems

3. Add widest_{real,complex}_dtype to dtype_helpers, use in test_special_cases