Function constants for Identity and No-op

[Peter van der Zee]

>> What's the issue with document.createElement('object')?

> It's a callable exotic object.

>> Function.isFunction? :D

> typeof is what you are looking for.

There is precedent (at least in IE [1]) for exotic functions where
`typeof` returned "unknown". Could happen for any exotic value unless
the spec changed on that. An `isFunction`, or rather, a simple
`isCallable`, may not be that far off the mark and is in line with the
existing `isArray`. Though I'd much rather have callables invariantly
locked down to being "typeof function". Even if that means explicit
exceptions to some legacy cases.

- peter

PS. Regexes in firefox were "callable" and had typeof function, but I
think that's so far back [2] it's not super relevant here. Of course
the same could be said about the IE case.

[1]; one of many examples:
http://stackoverflow.com/questions/10982739/typeof-returning-unknown-in-ie
[2]; https://bugzilla.mozilla.org/show_bug.cgi?id=61911