Fix newline handling/ground interaction.
Make 'let' return the last value it evaluates, add naive 'fn' implementation.

The naive 'fn' uses this augmented 'let' instead of a real lambda.
Add 'comment' type node.

'Comments' will blow up when evalled; I'm not sure whether to eliminate
them in the reader, or at one of several points in the reader, or to.
Add support for arrays and maps in the scala style preprinter.