Reorder expression of better accuracy

[IanBriggs]

I think there is a typo in sincos.h that dramatically increases error for the sin and cos functions by parenthesizing an expression the wrong way. You can see a more detailed exploration in https://github.com/IanBriggs/vdt_cos_error, but in short, in sincos.h, line 123 is:

c = 1.0 - zz * .5 + zz * zz * get_cos_px(zz);

This adds 1.0 and -zz * .5 together first, but it's more accurate to go from smallest to largest magnitude, like this:

c = 1.0 + zz * (-.5 + zz * get_cos_px(zz));

This cuts the worst case error approximately in half for fast_cos on the domain $[0, \frac{\pi}{2}]$.

Here's a plot of error values versus MPFR over this interval: