Add a FromJSONKey instance for Data.Fixed

[VictorCMiraldo]

We have a ToJSONKey (Fixed a) instance:

aeson/src/Data/Aeson/Types/ToJSON.hs

Lines 1432 to 1433 in 45d31f1

	instance HasResolution a => ToJSONKey (Fixed a) where
	toJSONKey = toJSONKeyTextEnc (E.scientificText . realToFrac)

But there's no corresponding FromJSONKey (Fixed a) available. Because we have a FromJSONKey Double, it seems like this was just never added and never needed. I now have some Maps with fixed-point numbers as their keys and therefore would benefit from a FromJSONKey (Fixed a) instance. A lot of the scientific parsing functionality is internal to aeson and not exported, so I can't just add it the orphan instance on my project and call it a day, plus, if I can fix it for everyone, than it's better for everyone! :)

I tried reusing the Scientific parsing as much as possible and checked it does indeed reject large exponents in a cabal repl:

ghci> x = Object (KM.fromList [("4",String "a"),("5.32e4300",String "b")])
ghci> fromJSON x :: Result (M.Map (Fixed E12) String)
Error "found a number with exponent 4298, but it must not be greater than 1024"