restructure this multi-crate project to use Cargo's "workspace" feature

this solves an issue in the Nix build, where managing multiple
Cargo.lock files is otherwise tricky. it causes (or fails to fix?) an adjacent issue where
the spirv builder doesn't seem to have everything it needs vendored.

crates/coremem/src/geom/mod.rs

@@ -0,0 +1,16 @@
+mod line;
+mod polygon;
+pub mod region;
+mod units;
+mod vec;
+mod vecu;
+
+pub use line::Line2d;
+pub use polygon::Polygon2d;
+pub use region::{
+    Cube, CylinderZ, Dilate, InvertedRegion, Memoize, Region, Sphere, Spiral, SwapXZ, SwapYZ, Torus, Translate, Union, WorldRegion, Wrap
+};
+pub use units::{Coord, Meters, OrdMeters, Index};
+pub use vec::{Vec2, Vec3};
+pub use vecu::Vec3u;
+