Add a global scope.

[mariari]

If a user writes some code like

(deflex custom-transfer
    (transfer :from-address #xFFFFFFFFF
              :to-address   #x111111111
              :amount       250))

at the top level, then they will just recieve a reference to a let binding that does not exist.

It would be good if when these functions are run without a proper circuit body to catch them that they are added to a global scope that can be compiled to VAMP-IR.

We can run dead code elimination if this costs performance.