Better compatibility with functional programming

[mickdekkers]

Currently, vakt.check always throws. This is a side-effect and thus undesirable for fp.

We could rename check to assert and add a non-throwing validation method.

Not sure on the exact API yet. For fp using an Either(Error, value) would probably be best, but it would add an unnecessary dependency and increase the bundle size even for users that only use the throwing version. Perhaps going the lodash route of vakt/fp or vakt/fantasy could be a solution?