RFC: Object parser

[phadej]

An applicative ObjectParser letting us make "strict" object parsers. cc @hvr

Performance is slightly degraded, as you can suspect:

OTOH in case of missing / extra keys parser will fail early.

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Motivation: for the TokenStream parser #560 such interface would be needed. The objectField could be a member of new class:

BTW, why parseJSON :: Value -> Parser a and not parseJSON :: Parser a, where

 newtype Parser a = Parser {
       runParser :: forall f r.
                    JSONPath
+                -> Value
                 -> Failure f r
                 -> Success a f r
                 -> f r
     }

(Probably because of manual instances, i.e. not using combinators like withObject)

-- | "dual" of 'KeyValue'
class OP p op | p -> op | op -> p where
    objectField :: Text -> p a -> op a
    runObjectParser :: String -> op a -> p a

instance OP Parser ObjectParaser where
    ... 

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EDIT: if aeson is a wrong place for this, I'll add it to aeson-extra.

TODO:

• documentation
• tests
• generics