cl-structures: Global (ew!) and local string interning!

- StringArena provides an arena for immutable strings, inspired by other string interners, and keeps track of the ends of every allocated string. Strings inserted into the arena are assigned a Symbol.
- intern::Interner keeps track of the hashes of each inserted string, and provides deduplication for interned strings. This allows referential comparison between interned strings
- global_intern::GlobalSym provides metered access to a Global Interner, and has a Display implementation which queries the Interner.

The global interner is planned for use in cl-ast.

TODO: the unstable raw_entry API is about to be removed from Rust. Maybe switch to hashbrown, or write my own hash table?

compiler/cl-structures/src/lib.rs

@@ -2,9 +2,17 @@
 //! - [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)]
+#![feature(
+    inline_const,
+    dropck_eyepatch,
+    decl_macro,
+    get_many_mut,
+    hash_raw_entry
+)]
 #![deny(unsafe_op_in_unsafe_fn)]
 
+pub mod arena;
+
 pub mod span;
 
 pub mod tree;