coremem_types: move Vec3u under the vec:: namespace

it's conceivable that Vec3u could be merged with Vec3 someday.
best to keep them in the same namespace, to make any such effort easier.

crates/spirv_backend/src/lib.rs

@@ -21,8 +21,7 @@ pub use support::{Optional, UnsizedArray};
 
 use mat::{IsoConductorOr, Ferroxcube3R1MH, FullyGenericMaterial, Material};
 
-use coremem_types::vecu::Vec3u;
-use coremem_types::vec::Vec3;
+use coremem_types::vec::{Vec3, Vec3u};
 
 type Iso3R1 = IsoConductorOr<Ferroxcube3R1MH>;