Function spawn 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn spawn(
    ctx: *mut UnrustContextWrapper,
    unity_entity: UnityEntity,
    inbuilt: *const InbuiltData,
    len: usize,
    custom: *const u8,
    custom_len: usize,
    custom_state: *const u8,
    custom_state_len: usize,
) -> u64