cl-structures: Use hashbrown's hash table implementation for deduplication.

compiler/cl-structures/src/lib.rs

@@ -2,13 +2,7 @@
 //! - [Span](struct@span::Span): Stores a start and end [Loc](struct@span::Loc)
 //! - [Loc](struct@span::Loc): Stores the index in a stream
 #![warn(clippy::all)]
-#![feature(
-    inline_const,
-    dropck_eyepatch,
-    decl_macro,
-    get_many_mut,
-    hash_raw_entry
-)]
+#![feature(inline_const, dropck_eyepatch, decl_macro, get_many_mut)]
 #![deny(unsafe_op_in_unsafe_fn)]
 
 pub mod arena;