Add anchors to every block-level element

[jdpage]

Reincarnation of #9, depends the block-formatter rewrite from #2. Every block-level element gets a fragment identifier, (possibly displayed in the margin; @woodrowbarlow thinks it would be too cluttered), to easily allow people to link to things.

• work out a way of constructing the fragment in a way that is stable/semi-stable across edits
• implement the update to the rendering engine
• stylesheet changes

[woodrowbarlow]

can i use U+2693 for these and hang them in the margin on the left instead of to the right of the element (including on headings)?

[jdpage]

I think the solution to the first problem may be to let the fragments be generated in the AST, and then somehow spewed out in the reconstituted markup in a non-intrusive way, so that when the AST is reconstituted from the markup, the fragment identifiers remain the same. Syntax suggestions welcome.