Revamped to use new Indenter system.

I am no longer convinced that this is an expressivity improvement, but at least
it's definitely a performance improvement, so we'll call that a win. Indenting
languages without a "real" parser is a fucking PITA. (Real is in scare quotes
there because you don't need a real parser, you actually need a parser that can
incrementally reparse on a line-by-line basis; good luck finding one of those!)