Simplify public-facing type names

[fjarri]

There are some elements of the public API that refer to their implementation and in general are a little too unwieldy. An example would be MessageBundle/VerifiedMessageBundle. These are better changed to something like Message/ProcessedMessage. This will mean that the current internal types containing "single" (echo or direct) messages will have to be renamed to not be confused with them (to SingleMessagePart etc?)