Local definitions: let bindings in terms and processes

[np]

A self-explanatory example:

let_example =
   let T = Int in
   let f = _+_ in
   proc(c : ?T.!T)
   recv c (x : T).
   let y = (f x x).
   send c y

A form with type annotation let x : T = t would be equivalent to let x = (t : T) using #18.