Remove decimal output from Disco

[byorgey]

The automatic decimal output is perhaps a misfeature. At the very least it is annoying that it cannot be controlled. If you want to see the decimal equivalent of something you can always, say, add 0.0 to force that output, but there is no way to do the opposite.

I don't think there's really any reason to have decimal output in a teaching language for discrete math. We should simply remove it. (However, we should keep the ability to parse rational numbers in decimal format.)

See also #400.