Let expression cannot occur within another expression

[mmatsa]

Let expressions can not be used within other expressions today.
Lionel asked me to add an issue for this.

This would support use like this:

rule MyRule()
→
Result(
  [binding ]->
  let #ans1 = OtherFunction(binding)
  let #ans2 = OtherOtherFunction(binding, #ans1)
  #ans2
  )

Today, without this fix, such use gets an error like this:

Error: Let expression cannot occur within another expression: it must be a the top-level expression.