cl-structures: Interning v3: ACTUALLY DO THE THING

Here we have *real* interning, producing unique references if and only if the input is unique! Boy am I glad I invested time into this, because it's really fun to work with.

Hopefully my logic regarding Send-ness and Sync-ness aren't completely unsound.

compiler/cl-structures/src/lib.rs

@@ -7,6 +7,8 @@
 
 pub mod arena;
 
+pub mod intern;
+
 pub mod span;
 
 pub mod tree;