Merge IncrementalEdit and SourceEdit

[ahoppen]

After #2527, IncrementalEdit and SourceEdit are more or less equivalent types. Both store a replacement range and a replacement. The only meaningful difference is that SourceEdit has String as a replacement and IncrementalEdit has an array of bytes as a replacement and can thus also represent invalid UTF-8. We should merge them, ideally into a type that is byte-based (because it’s more versatile) but that offers String-based accessors.

The best outcome would be if SourceEdit could be the type that’s used to represent the edits, IncrementalEdit becomes a deprecated typealias to SourceEdit and SourceEdit gains (possibly deprecated) members for all the members that IncrementalEdit had, for API compatibility.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://124118127