Versioning enum updates, rename "Cancelled" identifiers to "Canceled" (💥 BREAKING CHANGE), and README updates

[cretz]

What was changed

• Changed to use existing API enum for TaskReachability
• Changed VersioningIntent to be an enum
• Changed identifiers (but not text necessarily) from Cancelled to Canceled - 💥 BREAKING CHANGE
• Added a section in the README clarifying workflow inheritance
• Added a section in the README clarifying some details about the native shared library

Checklist

1. Closes Use single-L for all cancelled (sic) type names like .NET does #109
2. Closes [Feature Request] Clarify inheritance rules/reasons for workflow attributes #106
3. Closes [Feature Request] Clarify in README the role of the native shared library #107

[Sushisource]

Oh, so it looks like the exhaustive matches still work, which is all I cared about. When I did this before I was forced to add a blanket _ => xxx case to the switches which is why I didn't want to use enums.