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/Cargo.toml

@@ -9,3 +9,4 @@ publish.workspace = true
 
 [dependencies]
 hashbrown = { version = "0.14.3", default-features = false }
+cl-arena = { path = "../cl-arena" }