Hoist some predicates into vals.
Turn our TreeSet into a Seq ASAP.

Oh Scala, you go to such trouble to preserve bullshit like BitSetness, but you
fail miserably to preserve actually important properties, like sortedness.
Pass (Char,Syntax) to scan predicates.

Previously we were passing row and col so that one could turn around and look
up something like syntax. Now we'll just pass it directly. If you want to hide
sneaky stuff in the buffer: stick it in a syntax instance.
Make it easy to ask if syntax is "normal code".