Implement _Coerce :: TomlBiMap

[chshersh]

I noticed that it's sometimes useful to be able to create TomlBiMap for newtype if you already have a TomlBiMap for the type itself. For example:

newtype Portion = Portion
    { unPortion :: Double
    }

_Portion :: TomlBiMap Portion AnyValue
_Portion = _Coerce _Double

Currently, it's not straightforward to create custom TomlBiMaps for newtypes. So I think we can implement _Coerce to help with this. And, as a proof of concept, we can implement _BatmanDouble BiMap using _Coerce

tomland/test/Test/Toml/Codec/Combinator/Common.hs

Lines 37 to 40 in 4581781

	-- | Wrapper over 'Double' and 'Float' to be equal on @NaN@ values.
	newtype Batman a = Batman
	{ unBatman :: a
	} deriving stock (Show)

and get rid of testDouble for BiMap:

tomland/test/Test/Toml/Codec/BiMap/Conversion.hs

Lines 71 to 78 in 4581781

	-- Double needs a special test because NaN /= NaN
	testDouble :: PropertyT IO ()
	testDouble = hedgehog $ do
	x <- forAll G.genDouble
	if isNaN x
	then assert $
	fmap isNaN (forward B._Double x >>= backward B._Double) == Right True
	else (forward B._Double x >>= backward B._Double) === Right x