Next major iteration on Cipher interface

[bford]

This pull request represents the next iteration on the Cipher interface that I mentioned during the last meeting. It will require minor changes to all code that called the old cipher.Crypt method, to invoke the new cipher.Message or cipher.Partial methods instead. Partial is equivalent to the old cipher.Crypt(..., abstract.More{}), getting rid of the somewhat clunky use of variable-length options arguments to the workhorse message-processing methods.

The other significant API change is that the new Cipher interface no longer has a need for the "Direction" option-arguments (which was insufficient anyway as it didn't quite provide a proper stream-cipher mode of operation): instead, both the Message and Partial methods now take a third byte-slice argument, 'key', in which the caller explicitly specifies what - if any - input should be absorbed into the Cipher's state to key (or re-key) it for the next message (or hash/MAC output or whatever).

I've rewritten and greatly expanded the documentation for the Cipher interface, to include a bunch of examples of how to use it in various ways; please see that documentation for details on how to use it.

Currently some tests fail merely because the bits have changed in various test-cases; this will be fixed before merging this branch (and possibly after adding some more extensive Cipher tests; see the work-item I'll be posting shortly).