Trait-based error handling specification

[nicopap]

Fixes

• Closes Provide a way to send an event instead of logging #4 (with Emit)
• Closes Exclusive systems not supported by #[sysfail(log)] #3 (by using #[exclusive_sysfail])
• Closes Provide a way to exit application on error #2 (by using #[sysfail(Emit<AppExit>)])

Changelog

• Removed all deprecated methods
• Remove #[quick_sysfail] in favor of #[sysfail(Ignore)]
• Remove support for Option<()>. Consider replacing option? by option.ok(())?
• Replace the log = "foo" syntax by Log<ErrorType, Foo>.
• Automatically add return type to system. This means that you should remove
  the return type from your #[sysfail] systems.
• Failure is now a trait on the error type returned by the #[sysfail]
  system, rather than the whole return type.
• Failure now has an associated type: Param. It allows accessing arbitrary
  system parameters in the error handling code.
• Renamed FailureMode to Dedup.
• Now bevy_mod_sysfail directly depends on bevy, rather than its subcrates
• Added the full features, enabled by default. Disabling removes the bevy
  dependency, to only depend on bevy_ecs, but at the cost of removing
  the Log Failure definition.
• Added the Emit Failure, which sends Errs as bevy Events.
• Added LogSimply, a variant of Log that works without Res<Time>
• Added example showing how to extend with your own behavior the Failure trait.
• Added the system name to the log message's "target" field (by default, this is
  the bit of text before the "ERROR" colored text)
• Added #[exclusive_sysfail], works like #[sysfail] but is fully supported
  by exclusive systems. It only supports Failures where Param = ()

[nicopap]

I'm going to merge this next week.