refactor: convert Validator to Class

[alexlafroscia]

The "meat" of this change converts the existing Validator implementation into a class. Specifically, Validators are now Monoids that can be combined to create more complex validations:

This refactors the concept of what a Validator actually is, changing it from a Function into a Class. This allows for co-locating all of the information about how to check a value and format the error message into a single location.

The Validator class can be treated as a Monoid, meaning that it can be combined with other instances of itself to build up more complex Validators. Many operations that we supported previously can be thought of as combinations of other Validators, so I thought that formalizing that made sense. With base combinators like and and or, we can build up some of the type modifiers that we support like arrayOf or oneOf. All of these can be expressed as Validators, in terms of other Validators.

Note: This will conflict with some of the changes made in #88. I'll refactor whichever pull request lands later so that they're compatible.

[alexlafroscia]

I wanted to expose the base Validator class to allow users to extend it themselves and create custom validators, but I'm not sure that's possible with the current state of the babel-plugin-filter-imports. Those code trips up the plugin:

import { Validator } from '@ember-decorators/argument';

class SomeValidator extends Validator {}

With good enough dead code elimination, it should be safe to support this; we filter out the use of @argument everywhere, which removes all usage of SomeValidator, which means that Validator is no longer used and can also be pruned. However, this doesn't seem to be supported by babel-plugin-filter-imports at this time and would be easy to implement in the future, should we figure out how to support it.

[pzuraq]

👍 This looks good, I think the classes really clean things up in general, nice job!