Require client provide DeltaGenerator with domain.
Validate the domain.
Don't pass alphabet to getRandomString.
Whitespace.
Fix long lines in DeltaGenerator.
Define isInsert/isRetain on Op, not Delta.

The methods conceptually belong to Op, not Delta, and the implementation in
Delta was merely passing through to Op anyways. This is an api change.
Whitespace and long lines.
Whitespace.
Fix comment.