Factored Codex mode bits.

CodexReadingMode now provides the code styles and allows one to display a
buffer of Codex formatted text.

CodexUtil now handles formatSig &c.
Added codex-summarize-element.

This displays the javadocs and sig for a single element in a buffer instead of
a popup. Sometimes the data is too big for a popup, so one wants to dump it to
a buffer for easier perusal and searching.