gen_server: TimerHandle arm_after/cancel + Sys::Timer dispatch (RFC 015 §4.3, §5)

src/lib.rs

@@ -52,7 +52,7 @@ pub use channel::{
 };
 pub use gen_server::{
     call, cast, shutdown, whereis_server, CallError, CallTimeoutError, CastError, GenServer,
-    NamedServerBuilder, ServerBuilder, ServerCtx, ServerName, ServerRef, Watcher,
+    NamedServerBuilder, ServerBuilder, ServerCtx, ServerName, ServerRef, TimerHandle, Watcher,
 };
 pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};