Get rid of the `js` form

Having two different ways of calling javascript functions seems
annoying. This way, native functions like Math.sqrt and user-defined
functions can be called the same way. If the function uses `this` for
something, there will be problems though.

Binding Math.* in the global scope seems overkill now, since it can be
used directly:

    // Returns `3`
    [Math.sqrt, 9]