Notify debouncer

Tiny debouncer for notify. Filters incoming events and emits only one event per timeframe per file.

Features

• crossbeam enabled by default, for crossbeam channel support.
  This may create problems used in tokio environments. See #380.
  Use someting like notify-debouncer-mini = { version = "*", default-features = false } to disable it.
• serde for serde support of event types, off by default