Macro wren::wren_load_module_fn

macro_rules! wren_load_module_fn {
    ($f:path) => { ... };
    ($f:path, $alloc:path) => { ... };
}

Wrap a Fn(&mut VM, &str) -> Option<String> as an ffi-suitable LoadModuleFn.

Note: If a custom allocator is being used, it must be passed to this macro as well. This isn't required when using the default allocator.