List is too general

[jml]

GraphQL says lists are homogeneous. All elements should be the same type.

Our type is currently:

newtype List = List [Value] deriving (Eq, Ord, Show)

We could change it to:

data List
  = IntList [Int32]
  | FloatList [Double]
  -- ... and so forth

I don't know if this is a good idea or not, but worth experimenting after other bits are more coherent.