`defprop` is used to define properties on objects, and is the king of
mutability. It's useful though for low level mechanics in implementing
abstractions, and provides a cleaner API than `Object.defineProperties`.

`is(x, x)` could return false if `x` is an object

N.b.: If you do `Object.create(function() {}, {})`, the resulting object
will have a constructor set, but not to the prototype function and I have
no idea what indeed it gets set to. Might be worth investigating.

Hint: there is no coverage of `defprotocol`. Write some tests, man.

Damn gamification makes me want to achieve 100% coverage. I'm so easy..

This should probably change to something better, such as a **/*.api.js or
something such that it's possible to have files explicitly marked public
and others that would be considered private.

This probably needs tests.