allocator: add arena_allocator

This allocator uses Lua userdatas for dynamic allocation
that is automatically freed when the current scope exits.

src/arena_allocator.h

@@ -0,0 +1,25 @@
+/**
+ * An arena allocator using the Lua state; similar to luaL_Buffer.
+ * Initialize the arena with lxl_arena_init(), and you can use lxl_arena_malloc(),
+ * lxl_arena_zero() to allocate (and optionally zero) the memory.
+ * lxl_arena_free() can be optionally used to free memory, but this is generally not needed.
+ */
+
+#ifndef LUA_ALLOCATOR_H
+#define LUA_ALLOCATOR_H
+
+#include <lua.h>
+
+typedef struct lxl_arena {
+  lua_State *L;
+  int ref;
+} lxl_arena;
+
+void lxl_arena_init(lua_State *L, lxl_arena *arena);
+void *lxl_arena_malloc(lxl_arena *arena, size_t size);
+void *lxl_arena_zero(lxl_arena *arena, size_t size);
+char *lxl_arena_copy(lxl_arena *arena, void *ptr, size_t len);
+char *lxl_arena_strdup(lxl_arena *arena, const char *str);
+void lxl_arena_free(lxl_arena *arena, void *ptr);
+
+#endif