Add more Decoder helpers

[brandonchinn178]

toDecoder :: DecodeM a -> Decoder a
toDecoder m = makeDecoder (const m)

applyDecoder :: Decoder a -> Value -> Decoder a
applyDecoder decoder = toDecoder . runDecoder decoder

-- Type mismatch: Expected String, got: <value>
typeMismatch :: Text -> Decoder a
typeMismatch' :: Text -> Value -> DecodeM a

-- Invalid value: <msg>, got: <value>
invalidValue :: Text -> Decoder a
invalidValue' :: Text -> Value -> DecodeM a

hide DecodeM functions; recommend invalidValue or typeMismatch over fail