Add `hann` export to be equal to `hanning`

[staticfloat]

Since the original author's name was "Julius Von Hann", and referring to it as "hanning" is done only to draw similarities to the similar "hamming" window, let's export it under both common names.

[rob-luke]

Sorry for string reviewing. But could you please add a small test?

[ViralBShah]

Good to merge?