fourier_transform_functions.py -> fft.py, update respective uses (#504)

Author: honno

spec/API_specification/array_api/__init__.py

@@ -12,6 +12,7 @@
 from .statistical_functions import *
 from .utility_functions import *
 from . import linalg
+from . import fft
 
 
 __array_api_version__: str = "YYYY.MM"

spec/API_specification/array_api/fourier_transform_functions.py renamed to spec/API_specification/array_api/fft.py

@@ -15,7 +15,7 @@ A conforming implementation of the array API standard must provide and support t
 Objects in API
 --------------
 
-.. currentmodule:: array_api.fourier_transform_functions
+.. currentmodule:: array_api.fft
 
 ..
   NOTE: please keep the functions and their inverse together