Add `withTypeableObject` that extracts name of the object from the type

[chshersh]

Usually when I have custom data type like this one:

data Foo = Foo
    { fooBar :: Int
    , fooBaz :: Double
    }

I write FromJSON instance like this:

instance FromJSON Foo where
    parseJSON = withObject "Foo" $ \o -> ...

I need to specify name of the type to get good error messages. But if type name changes, I need to not forget to update this function. Also, it's not really convenient to write name of type each type. I propose to add the following function to the aeson library:

typeName :: forall a . Typeable a => Text
typeName = show $ typeRep $ Proxy @a

withTypeableObject :: forall a . Typeable a => (Object -> Parser a) -> Value -> Parser a
withTypeableObject = withObject (typeName @a)