438: Angle: add the normalize_zero method r=kvark a=mathstuf

This method is like `normalize` except that it normalizes to have an
absolute value of no more than `turn_div_2`.

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This is useful for making sure that an angle is no more than some offset from a target angle (e.g., implementing maximum turn rates in games).

Co-authored-by: Ben Boeckel <mathstuf@gmail.com>