Add more combinators and provide a better API

This pre-release for the unstable API gets us closer to the first stable release that shall happen soon.

Add basic combinators for composition

feature: Add new adapter for function composition using fmap

The wrapper type: types::function_wrapper, which can be built via
helper function fn, allows the usual composition with fmap to be
applied to functions as well.

So, given functions fx: A -> B and fy: B -> C, by wrapping both
around types::function_wrapper, we can fmap(fx, fy) that composes
fx and fy, so by providing an argument x of type A, we have:

fmap(fx, fy)(x) == fy(fx(x))