Remove 'abort' feature

[ccremer]

Summary

It never felt quite right to have it in this lib (and in practice I never had a use case).
Users can perfectly return their own kind of errors and check for those with errors.Is().

The Result now has removed the following funcs:

• IsCompleted() -> Without the abort feature it's equivalent to ÌsSuccessful(), use this instead
• IsAborted() -> removed, return your own error type and check for this using errors.Is()

Checklist

• Categorize the PR by setting a good title and adding one of the labels:
  fix, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Update documentation.
• Update tests.