Idea: Better `Char` and `String` Fuzzers

As mentioned in #89, I'm a fan of [`hedgehog`](https://hackage.haskell.org/package/hedgehog).

`hedgehog` comes with a useful set of [generators for characters](https://hackage.haskell.org/package/hedgehog-0.6.1/docs/Hedgehog-Gen.html#g:7) and for [strings too](https://hackage.haskell.org/package/hedgehog-0.6.1/docs/Hedgehog-Gen.html#g:8), such as (as would be written in Elm):
* `lower : Fuzzer Char`
* `alphaNum: Fuzzer Char`

These are great building blocks for larger fuzzers, as the generator for strings has the form (ignoring `Range`, discussed separately in #89):
```elm
string : Fuzzer Char -> Fuzzer String
```
Thus allowing the user to choose what class of characters to use when generating a ~list~ string.
In many cases all printable Unicode characters are invalid input.

Even if you represented valid strings _safely_ using `type MyAlphaString = MyAlphaString String`. Writing a fuzzer for `MyAlphaString` is not straightforward.

Would you accept a PR in this direction? or prefer it as a separate package?