IMO, we shouldn't be introducing distinct EditText/EditMarkup
methods because it just doesn't make sense, you can do both in
one and EditCaption is a pretty odd case so we live it be.
- Callback structs has been moved to callbacks.go
- User / Chat and its methods -> chat.go
- Concept of Editable and Message Signature introduced
- No more types.go, it's been refactored into media.go + friends
This commit refactors lots of duplicated code from bot.go, so
instead of having N redundant SendX methods, now it's the
responsibility of all Sendable objects to implement Send(..)
instead. Impl in types_send.go, 150 LOC only!
