Binder refactor

[janmasrovira]

Important changes:

1. Remove BinderInfo and replace it with an explicit field of type Binder in the nodes where it makes sense. Currently Binder is only a name and a type. We may need to extend it or make it dynamically extensible in the future.
2. Node' is now also parameterized by the type in its binders.
3. Refactor destruct to make it easier to understand and less error prone (hopefully).
   Before we had the following fields in NodeDetails:

        -- | 'nodeChildren' are the children, in a fixed order, i.e., the immediate
        -- recursive subnodes
       _nodeChildren :: [Node],
        -- | 'nodeChildBindersNum' is the number of binders introduced for each
        -- child in the parent node. Same length and order as in `nodeChildren`.
        _nodeChildBindersNum :: [Int],
        -- | 'nodeChildBindersInfo' is information about binders for each child, if
        -- present. Same length and order as in `nodeChildren`.
        _nodeChildBindersInfo :: [[Info]],

   All the three list need to match their length.
   I have introduced a new data type NodeChild that has the same information as before but grouped nicely. All the previous fields are now merged in a single list:

       _nodeChildren :: [NodeChild]

   where

   data NodeChild = NodeChild {
   -- | immediate child of some node
   _childNode :: Node,
   -- | Binders introduced by the child
   _childBinders :: [Binder],
   -- | length of `_childBinders`
   _childBindersNum :: Int
   }

Other changes:

1. Introduce types LambdaLhs and PiLhs, which are useful for unfolding and folding.
2. The semicolon after the last item in a letrec is no longer optional for consistency.