rsevents-extra 0.2.0

This release upgrades to the latest version (0.3.1) of rsevents, and implements the newly updated and expanded rsevents::Awaitable trait ((see the rsevents 0.3.x changelog for more info)[https://github.com/neosmart/rsevents/releases/tag/0.3.1]).

• The biggest feature of this release is a new Semaphore type, which is an attempt at modeling a truly safe semaphore for rust (something lacking from the standard library). Semaphore is built on an rsevents auto-reset event, and uses that under-the-hood to efficiently wait for a Semaphore to become available when its concurrency limit has been reached.
• The new Semaphore type attempts to bridge the "standard" usage of a sempahore wherein the concurrency count is manually incremented (when the semaphore is "released") with rust's concepts of ownership and RAII-friendly guard types. Please provide feedback on your usage of Semaphore so we can refine the API together!
• The rsevents_extra types are now static friendly with const fn new() initializers so they can be declared and used as global/local static variables without requiring the usage of lazy_static or once_cell.