add gcd operation? are the signatures the way we want?

[blerner]

Just adding a num-gcd method to Pyret since it's missing. But do we need the array argument to jsnums.gcd, since we never even called that function prior to this PR, and we're only calling it with a single argument here? @ds26gte did you need that array for spyret or wescheme or some other racket support?

[jpolitz]

This needs some tests in test-numbers so we can make sure it doesn't regress in the future.