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/dropless_arena/tests.rs

@@ -0,0 +1,42 @@
+use super::DroplessArena;
+    extern crate std;
+    use core::alloc::Layout;
+    use std::{prelude::rust_2021::*, vec};
+
+    #[test]
+    fn alloc_raw() {
+        let arena = DroplessArena::new();
+        let bytes = arena.alloc_raw(Layout::for_value(&0u128));
+        let byte2 = arena.alloc_raw(Layout::for_value(&0u128));
+
+        assert_ne!(bytes, byte2);
+    }
+
+    #[test]
+    fn alloc() {
+        let arena = DroplessArena::new();
+        let mut allocations = vec![];
+        for i in 0..0x400 {
+            allocations.push(arena.alloc(i));
+        }
+    }
+
+    #[test]
+    fn alloc_strings() {
+        const KW: &[&str] = &["pub", "mut", "fn", "mod", "conlang", "sidon", "🦈"];
+        let arena = DroplessArena::new();
+        let mut allocations = vec![];
+        for _ in 0..100 {
+            for kw in KW {
+                allocations.push(arena.alloc_str(kw));
+            }
+        }
+    }
+
+    #[test]
+    #[should_panic]
+    fn alloc_zsts() {
+        struct Zst;
+        let arena = DroplessArena::new();
+        arena.alloc(Zst);
+    }