Pod6 improvements

This is WIP, but it will give benefits right away, such as correctly working «...» as well as balanced formattingcodes like C< foo<bar> >.

It will also turn "output" blocks into preformatted blocks.

Things still missing:

formattingcodes really ought to use a nibbler and there needs to be a mechanism that applies formattingcodes *after* a whole paragraph is made, such as when building a table from multiple rows.