cl-arena: Add arena allocator implementation based HEAVILY on rustc-arena

It seems to pass the most basic of tests in miri (shame it needs the unstable `strict_provenance` feature to do so, but eh.)

compiler/cl-arena/src/typed_arena/tests.rs

@@ -0,0 +1,61 @@
+use super::TypedArena;
+    extern crate std;
+    use std::{prelude::rust_2021::*, print, vec};
+    #[test]
+    fn pushing_to_arena() {
+        let arena = TypedArena::new();
+        let foo = arena.alloc("foo");
+        let bar = arena.alloc("bar");
+        let baz = arena.alloc("baz");
+
+        assert_eq!("foo", *foo);
+        assert_eq!("bar", *bar);
+        assert_eq!("baz", *baz);
+    }
+
+    #[test]
+    fn pushing_vecs_to_arena() {
+        let arena = TypedArena::new();
+
+        let foo = arena.alloc(vec!["foo"]);
+        let bar = arena.alloc(vec!["bar"]);
+        let baz = arena.alloc(vec!["baz"]);
+
+        assert_eq!("foo", foo[0]);
+        assert_eq!("bar", bar[0]);
+        assert_eq!("baz", baz[0]);
+    }
+
+    #[test]
+    fn pushing_zsts() {
+        struct ZeroSized;
+        impl Drop for ZeroSized {
+            fn drop(&mut self) {
+                print!("")
+            }
+        }
+
+        let arena = TypedArena::new();
+
+        for _ in 0..0x100 {
+            arena.alloc(ZeroSized);
+        }
+    }
+
+    #[test]
+    fn pushing_nodrop_zsts() {
+        struct ZeroSized;
+        let arena = TypedArena::new();
+
+        for _ in 0..0x1000 {
+            arena.alloc(ZeroSized);
+        }
+    }
+    #[test]
+    fn resize() {
+        let arena = TypedArena::new();
+
+        for _ in 0..0x780 {
+            arena.alloc(0u128);
+        }
+    }