Improve the accuracy of real acos

[pearu]

As in the title:

-| acos | float32 | 548396 | 444291 | 7072 | 242 | - | - |
-| acos | float64 | 985930 | 12727 | 1338 | 6 | - | - |
+| acos | float32 | 961608 | 38291 | 99 | 3 | - | - |
+| acos | float64 | 992582 | 7416 | 3 | - | - | - |

The trick is to use identity arccos(x) = arctan2(sqrt(1 - x*x), x). Btw, this is the formula to which Hull et al algorithm also reduces when restricting to the real line.