circe-display-handler: Allow lists in addition to functions.

This replaces the old `circe-format-strings` hack by making the
format specifiers first class citizens as display handlers. The
former format strings list is now used to set display handlers
directly. A number of other code parts have been simplified a lot.
circe-server-handler: Rework for more clarity.

This change cleans up some crufty code. As the most visible change,
there's now a `circe-message-handler-table` which stores mappings
of commands to lists of handlers, much like `circe-display-table`
stores mappings of commands to singular display handlers. This
makes `circe-receive-message-functions` much less used as the general
go-to point for everything.
Cleanup of circe.el

Moving around various code pieces to make the organization of the
file make more sense.
circe-version: Add devel string.