Add in a new core language file.

 This will let us implement common functions seen in other lisps,
 and allow them to be importable, without explicit imports. The goal
 is to keep this as small as we can; we don't want too much magic.

 I've added `take' and `drop' as examples of what we can do.
Merge pull request #202 from paultag/paultag/feature/start-language-lib

Add in a language lib.