allow RepaintSignal: ?Sync

[zetanumbers]

Closes #1186.

[emilk]

Is all this new code just to avoid the performance impact of locking a mutex when the users requests a repaint? Or is there some other benefit I'm not seeing?

[zetanumbers]

Is all this new code just to avoid the performance impact of locking a mutex when the users requests a repaint? Or is there some other benefit I'm not seeing?

This PR firstly fixes logic of the library and only then the performance. There are several of benefits to it:

• Avoids mutex poisoning and panic deadlocks (i. e. if some thread panics while using request_repaint, then other threads would panic on a request_repaint too);
• Use of DST eliminates an indirection;
• Avoids unnecessary locks (majority of channel Senders (like mpsc) are Send + !Sync which are used by window handling libraries);
• There may be other downsides of relying on mutex, like if it's not implemented for some platform or another indirection (posix targets store mutex on the heap IIRC).