v0.10.0

This release brings a lot of deep changes to calloop, the highlights being:

Error handling is revamped: The error handling machinery of calloop has been completely revamped (thanks to @detly). A calloop::Error type is introduced, and the EventSource trait now has an Error associated type. Each event source now has its own error type.

Timer sources rework: Timer event sources are completely reworked, making them more flexible and integrating them directly into the event loop core logic, thus avoiding to spawn a background thread to manage them. This also introduces a "high-precision mode" for the event loop allowing sub-millisecond accuracy for the timers, it is obtained by initializing it with the try_new_high_precision() method.

Several other changes were introduced in this release:

• The MSRV is bumped to 1.53.0
• The generic::Fd adapter is now removed, as RawFd implements AsRawFd, making Generic<RawFd> usable.
• The internal logic of calloop has been reworked around a slotmap, making the RegistrationToken now Copy. This causes a breaking change to the low-level Poll API.
• The PingSource event source now uses an eventfd instead of a pipe on Linux (thanks to @detly).