Add `fromJson` and `toJson` about purescript-argonaut-codecs

[wclr]

fromJson that gets String and toJson that outputs String, this seems to be a bit misleading naming.

[sigma-andex]

I think it is only misleading if you think in terms of Argonaut where you have this intermediate Json type.
If you are coming from JS it makes perfect sense: fromJson converts from Json (which is a string) to an object and toJson converts an object to a Json (string). The JS equivalents would be JSON.parse and JSON.stringify()but unfortunately these are already taken.

[wclr]

I also think that such helpers are not needed to be supplied with the library just to shortcut parseJson >=> decodeJson, one can do it in its code if one wants the shortcut.