(Re-)introducing Syntax.

Every character position now has a Syntax instance associated with it along
with a Styles instance. Syntax allows code to know basic syntax information
about the entire file without resorting to reverse engineering it from Styles
(which is error prone/lossy anyway).

Presently the only two things we care about are: is this a comment? and is this
a literal of some sort? This will be used by blocker and indenters to avoid
spuriously matching things inside comments, &c.

A mode can also use syntax for its own more sophisticated purposes, if desired,
as long as it properly expresses the isComment and isLiteral nature of it
special syntax instances.
Report whether we revalidated on setViz.
Some temporary debug info.

Things start to slow down substantially after a while. It doesn't seem like a
memory leak, since we're not GCing a lot, but this debugging info also
indicates that it doesn't seem to be a ton of time spent in layoutChildren.

So I'm not sure what's baking who's noodle, but I'm leaving this
instrumentation in until it's all sorted.