second pass at shrinking the API

[slingamn]

This is looking better:

https://pkg.go.dev/github.com/ergochat/readline@v0.0.3

but there's still a lot of cruft. For example:

1. Listener should probably become a function pointer type rather than an interface, eliminating the need for DumpListener. See racey redundant prompt in ReadPassword methods #6 for discussion of the listener API
2. The relationship between PrefixCompleter, PrefixCompleterInterface, and functions like Do and Print is confused
3. Instance has too many methods
4. Painter should be a function pointer type, not an interface type
5. TabCompleter should probably be hidden
6. What are SegmentComplete and SegmentCompleter?

[slingamn]

What is (Config).UniqueEditLine?

[slingamn]

Effectively resolved by 0.1.0

[wader]

Thanks for all the work on this!