Mark Module as Send

[cspiegel]

Because it includes the raw pointer *mut openmpt_sys::openmpt_module, Module is not automatically Send. However, as far as I can tell, it's valid for it to implement Send. According to the C API docs:

• libopenmpt is thread-aware.
• Individual libopenmpt objects are not thread-safe.
• libopenmpt itself does not spawn any user-visible threads but may spawn threads for internal use.
• You must ensure to only ever access a particular libopenmpt object from a single thread at a time.
• Consecutive accesses can happen from different threads.
• Different objects can be accessed concurrently from different threads.

So it's safe to send OpenMPT pointers to different threads, as long as access is guarded, e.g. with a mutex. So Sync wouldn't be good, but Send is just fine, and will allow Module to be wrapped in a mutex.