e.g. `F_PI` is also available as `d::PI` and `f::PI` as `double` or
`float` respectively.
Most of our uses of those constants are `float` and it was error prone
and cumbersome to have to cast to that each time we used them.
The `F_` constants are intended to mimic `math.h`, however these new
constants are not, and all the "n over" constants are named
`N_OVER_` instead of `N_`, which helps avoiding the confusion that e.g.:
`F_2_PI` is actually `2/pi` and not `2*pi`.
Also added `F_TAU` which is 2 * PI.
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