The purpose of the Variables function is to tell a calling application
what symbols need to be present in the _root_ scope, so it would be
unhelpful to include child scope traversals. Child scopes are populated
by the nodes that create them, and are thus not interesting to the
calling application (for this purpose, at least).
The ForExpr is essentially a list/map comprehension, allowing projecting
one expression into another. From a syntactic standpoint it's the most
complex structure we've dealt with so far, with many separate parts.
The tests introduced here are not exhaustive but illustrate that the
basic mechanism is working.
This function is effectively the implementation of Variables for all
expressions, but unfortunately we still need to declare a wrapper around
it as a method on every single expression type.
