Fixes effectfully#51, effectfully#52

We make the following changes to the AST by adding
- a newly introduced a top-level `Program` node, which is parent to
- a newly introduced `Statements` node, which is parent to
- a `Statement` node, which is chaned to be a parent of
- an `Expr` node.

We make the following changes to Statement by
- introduce a `EFor` node to the typed AST.

We update update the evaluator by
- providing an explicit transformer stack (`EvalT`),
- fixing the semantics for for loops, and
- switching to CPS both in evaluator and normaliser.

We also add minor improvements to the codebase by
- fixing `Sign` vs `Sig` naming issue,
- cleaning up some naming conventions, and
- providing `stack bench` benchmark for testing generators for the new
  AST.