Expr/stmt eval that provides types

[agle]

Eval statement now uses new ExprEvalWithType which pushes types up the ast and passes the type to the ValueAbstraction: see https://github.com/agle/bincaml/pull/26/changes#diff-fe7cc53407f35da17c26c9fc0f79c4317c9083dcc897c2e0c0ffba47f5374712

I think this makes sense as a default because the semantics are defined by the type (bitvector width). There would be other sensible approaches though

• store the type attached to every expr tree node
• monomorphise the set of ops to include width arguments