Move vec3.rs -> vec.rs in preparation for adding a 2d vec

src/render.rs

@@ -3,7 +3,7 @@ use crate::geom::Point;
 use crate::{flt::{Flt, Real}, Material as _};
 use crate::mat;
 use crate::sim::{Cell, GenericSim};
-use crate::vec3::Vec3;
+use crate::vec::Vec3;
 use crate::meas::AbstractMeasurement;
 use font8x8::{BASIC_FONTS, GREEK_FONTS, UnicodeFonts as _};
 use log::trace;