Traversal over immediate subexpressions and the embedded values

[mmhat]

This PR adds a variant of Dhall.Core.subExpressions which allows you to map over the embedded values simultaneously.
It is useful when you need to work with the AST directly. Specifically, it enables functions like:

traverseExpr :: Monad m => (Expr Src Void -> m (Maybe a)) -> Expr Src Void -> m (Expr Src a)
traverseExpr f expr = do
    mx <- f expr
    case mx of
        Nothing -> subExpressionsWith absurd (traverseExpr f) expr
        Just x -> pure $ Dhall.Core.Embed x