math: Add PositiveSign type.

This will be used to enforce stronger restrictions on color components,
which will reduce the number of edge cases color processing code must
consider, and remove the the hazard of distinct negative zero
(<#537>).

all-is-cubes-base/src/math.rs

@@ -30,6 +30,8 @@ mod grid_iter;
 pub use grid_iter::*;
 mod rigid;
 pub use rigid::*;
+mod restricted_number;
+pub use restricted_number::*;
 mod matrix;
 pub use matrix::*;
 mod octant;