Improve logging and refactor AST utilities

[Omikhleia]

I had planned these for some time:

• refactor: AST utilities are in SU.ast in SILE 0.15-develop and silex.ast provides our compatibility layer for these with SILE 0.14.x, so it an obvious change for the future.
• fix: Improve error handling in Djot to report location details

The latter greatly improves debugging... Now we get line and colum, and stack details:

! Ignoring unattached attribute near [[…x {.loose}]] at temp/test.dj: in <snippet>:
   	[[{lang="en-GB"}␤:::␤␤Later on...␤x {.loose}␤␤:toto:␤␤--- "Hello world"[^1]␤␤--- Lorem ipsum dolor con]]

! Symbol 'toto' was not expanded (no corresponding metadata found) at temp/test.dj:7:1: in symbol

! Failure to find footnote '1' at temp/test.dj:9:18: in footnote_reference

error summary:
   Processing at: temp/test.dj:9:18: in footnote_reference

Or with -t (snippet):

! Symbol 'toto' was not expanded (no corresponding metadata found) at:
	temp/test.dj:7:1: in symbol
	temp/test.dj:7:1: in para
	temp/test.dj:2:1: in div
	temp/test.dj: in <snippet>:
		[[{lang="en-GB"}␤:::␤␤Later on...␤x {.loose}␤␤:toto:␤␤--- "Hello world"[^1]␤␤--- Lorem ipsum dolor con]]

On the first parsing warning, we don't get line and column details ('cause jgm/djot.lua#14), but at least we can extract and show some bits of text to help with finding and fixing them.