Cleaned up other docs in preparation for alpha-testing announcement
Created `math/utils' module for stuff that doesn't go anywhere else (e.g.
FFT scaling convention, max-math-threads parameters)
Reduced the number of macros that expand to applications of `array-map'
Added `flvector-sum', defined `flsum' in terms of it
Reduced the number of pointwise `flvector', `flarray' and `fcarray' operations
Reworked `inline-build-flvector' and `inline-flvector-map' to be faster and
expand to less code in both typed and untyped Racket
Redefined conversions like `list->flvector' in terms of for loops (can do
it now that TR has working `for/flvector:', etc.)
