Function constants for Identity and No-op

[Kevin Reid]

[no quotes because I'm not replying to anyone in particular]

An advantage that has not been mentioned yet, of having a canonical
function instance for particular behaviors, is that it allows for some
library-level optimization by being able to know what a function does
(which is otherwise opaque). For a simple example:

SomeKindOfImmutableCollection.prototype.map = function (f) {
    if (f === Function.IDENTITY) {
        return this;
    }
    ...build a new collection with f applied to elements and return it...
};

Of course, it would be silly to write coll.map(Function.IDENTITY), but a
caller might be passing the function from somewhere else.

(I only intend to point out this one benefit, not to claim it justifies the
feature entirely.)
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