DatatypeContexts is deprecated

[meimisaki]

I find that DatatypeContexts appear several times in this project.
(i.e. Point2/3, Vector2/3 and MergeableRecord)

According to this proposal: https://ghc.haskell.org/trac/haskell-prime/wiki/NoDatatypeContexts
DatatypeContexts is deprecated.
It was widely considered a misfeature, and has been removed from the Haskell language.
Although cabal-install makes Yampa buildable even in the latest GHC(by turning the flag on automatically).

Instead, we should use ExistentialQuantification(or GADTs) to add constraints on constructors.
For typeclass deriving, we could enable the StandaloneDeriving for convenience.

Well, actually I'm not sure if DatatypeContexts was necessary in this case.
I'll apologize if I misunderstood the usage here.

[ivanperez-keera]

Hi meimisaki

I agree.

In the future, Point2/3, Vector 2/3 and MergeableRecord should be removed from Yampa completely and other libraries used instead.

Would you be kind enough to change it by removing the datatype contexts wherever possible and send me a pull request?