core build system and array, it compiles!

base_arena.cpp

@@ -131,7 +131,7 @@ arena_from_buffer(void *buffer, size_t size) {
 
 struct Scoped_Arena {
     Arena *arena;
-    int pos;
+    size_t pos;
     Scoped_Arena(Arena *arena) {
         this->arena = arena;
         this->pos = arena->len;