Generalized formatting of generated values

Following a quick discussion from #324, this issue is being created to provide generalized formatting for values from a `Gen<'a>`.

Ideally, we'd change the `Gen<'a>` type to be a record, and add a `formatter` field that looks like `'a -> string`. To maintain compatibility, this field should have a default value of `sprintf "%A"`. Another function should be exposed to override this default, `withFormatter : ('a -> string) -> Gen<'a> -> Gen<'a>`.

Any values from a generator that are formatted should use the provided formatter.