Move header files to include/

include/arena.h

@@ -0,0 +1,17 @@
+
+#ifndef ARENA_H
+#define ARENA_H
+
+#include <stddef.h> // ptrdiff_t
+
+typedef struct arena {
+    void *begin,
+        *next;
+    size_t cap;
+} __attribute__((aligned(sizeof(void*)))) arena_t;
+
+arena_t* arena_new();
+void arena_reset(arena_t* a);
+void* arena_alloc(arena_t* a, size_t len);
+
+#endif