added constant metadata tag in the definition of constant variables

[danhammer]

I've only done this in core.clj. There may be other candidates, but I am not sure how this tag will interact with cascalog. Next step: run the tests after (set! *warn-on-reflection* true). See if there are strange reflections. The metadata tag will avoid undue reflections, as described here.

[eightysteele]

Very nice dude. +1. I have a local branch that totally cleans up testing so I'll fold this in and run tests after setting the reflection tag. #shipit