Add FromDhall instances for {Int,Word}{,8,16,32,64}

[Gabriella439]

Fixes #2007

[german1608]

Really cool! That TypeApplication extension fitted perfectly here.

[sjakobi]

Does the fact that the added decoders can fail after type-checking actually matter in practice?! If so, it should possibly be highlighted in the haddocks.

[Gabriella439]

@sjakobi: Yeah, this is a departure from my previous policy of only providing Decoders that could never fail if the type matched. The thing that tipped me over the edge was that we already do this for set and also I'm reasonably confident that our users are already doing this in the wild, too (e.g. for dates and IP addresses)