Track JSONPath in autoderived parsers. Fixes #357

[Lysxia]

The first commit is not a direct fix, but gets rid of some redundancy in error messages in nested records.

In the definition of (.:) (as well as its siblings (.:?), etc.) the location information is provided twice.

    data X = {a :: (Int,Int,Int)}
    data Y = {x :: X}
    instance FromJSON X where
        parseJSON (Object obj) = X <$> obj .: "a"
    instance FromJSON Y where
        parseJSON (Object obj) = Y <$> obj .: "x"

This results in this kind of error when using decodeEither:

    > eitherDecode "{\"x\":{\"a\": [1,2,true]}}" :: Either String Y
    Left "Error in $.x.a[2]: failed to parse field x: failed to parse field a: expected Int, encountered Boolean"

The location is given in two different forms; the second one is extremely verbose and also incomplete, missing the [2].

This commit simply removes the use of modifyFailure in the combinators, so that only the compact path description is left.

Left "Error in $.x.a[2]: expected Int, encountered Boolean"

One thing that might be problematic with this solution is that the function parse :: (a -> Parser b) -> a -> Result b no longer has any location information by default since it discards the JSONPath parameter. OTOH some users might prefer to insert modifyFailure themselves to customize their error messages, which previously prevented the use of (.:).

[Lysxia]

Oh wait, the Generic is still wrong.

[Lysxia]

This is why I should test before submitting the PR.

[Lysxia]

Fixes #357

[bergmark]

Thank you for this! Looks very nice.