Move list segmentation from ast.lua to blocks.lua?

[jgm]

Currently we don't emit match objects for lists, only list items. List items are gathered together into lists in the AST building phase. I dimly recall there was some reason for this, but it would simplify AST building, and make the match stream more useful, if the work were done at the earlier stage.

[jgm]

Similarly, providing a "link" match around "linktext" + "definition"/"reference" would be helpful.