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/support.rs

@@ -1,4 +1,4 @@
-use coremem_types::vecu::Vec3u;
+use coremem_types::vec::Vec3u;
 
 /// This is a spirv-compatible option type.
 /// The native rust Option type produces invalid spirv due to its enum nature; this custom option