bimap : (a -@ x) -@ ((b -@ y) -@ (LPair a b -@ LPair x y))
A linear bimap on linear pairs.
There is no general Bifunctor interface because it would not be implementable with
The same type signature consistently, for example LEither does not consume both
`f` and `g` linearly.
mapFst : (a -@ x) -@ (LPair a b -@ LPair x b)
mapSnd : (b -@ y) -@ (LPair a b -@ LPair a y)