Introduce read decoder and add note about rational decoder for Double

[chshersh]

It was proposed to add rational decoder in #22 to improve accuracy of decoding Double values. Unfortunately, this decoder also doesn't work. There's 6 years old issue in text library regarding rational decoder:

• Data.Text.Read.rational no more accurate than Data.Text.Read.double haskell/text#34

I was quite surprised to find this issue in text library because I was assuming that rational decoder should solve all problems with decoding Double. But my hedgehog property-based tests fail when I use rational decoder. The Encoder works properly though 👍

-           1.0000000000034547e-2
+           1.0000000000034549e-2

As a dirty and quick solution I'm using Read instance for Double to parse values and it works without any problems 👍 So I propose to add read decoder to sv library and add note to rational and double decoders about their accuracy.

This is the decoder (initially implemented by @vrom911, we're using it for Double and UTCTime at the moment and it works like a charm 😊):

readDecoder :: forall a . (Read a, Typeable a) => Decode' ByteString a
readDecoder = contents >>== \c -> maybe
    (badDecode $ errorMsg c)
    pure
    (readMaybe @a $ decodeUtf8 c) 
  where
    errorMsg :: ByteString -> ByteString
    errorMsg c = "Expect '" <> typeName @a <> "' but couldn't parse: " <> c

[gwils]

Thanks for letting me know about this. I'd like to get something like that into the library. I'd like for the user to be able to make the error on their own, either using the Bytestring or not.

Off the top of my head, something like the following (possibly with more general types)

read :: Read a => Decode' Bytestring a
read' :: Read a => (ByteString -> DecodeError' ByteString) ->  Decode' a

I will get around to this eventually. I'd accept a pull request much sooner :) although I wouldn't want TypeApplications like you're using there since I'd like to keep supporting GHC 7.10 for the time being.

[gwils]

It's also worth nothing that for doubles cassava uses attoparsec which in turn uses scientific http://hackage.haskell.org/package/cassava-0.5.1.0/docs/src/Data.Csv.Conversion.html#parseDouble
https://hackage.haskell.org/package/attoparsec-0.13.2.2/docs/src/Data.Attoparsec.ByteString.Char8.html#double

[gwils]

I'm playing around with encoding and liking double-conversion