Fix possible instant subtraction panic racing the clock

[alexheretic]

I got a panic that I found difficult to reproduce reliably. Instant subtractions will panic if the rhs is bigger, and two calls of Instant::now may be different. So its sort of a race with the clock to see if this panics.

thread '<unnamed>' panicked at 'specified instant was later than self', src/libstd/sys/unix/time.rs:292:17
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   5: std::panicking::begin_panic
             at src/libstd/panicking.rs:412
   6: std::sys::unix::time::inner::Instant::sub_instant::{{closure}}
             at src/libstd/sys/unix/time.rs:292
   7: <std::time::Instant as core::ops::arith::Sub>::sub
             at /rustc/d1731801163df1d3a8d4ddfa68adac2ec833ef7f/src/libcore/result.rs:764
             at src/libstd/sys/unix/time.rs:291
             at src/libstd/time.rs:218
             at src/libstd/time.rs:307
   8: notify::debounce::timer::ScheduleWorker::run
             at /home/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/notify-4.0.9/src/debounce/timer.rs:99

[passcod]

Great find!

[matklad]

Would appreciate a bugfix release with this fix :)

[passcod]

Done in 4.0.10.