it's acceptable to have "Variables" set to nil in an EvalContext, if
a particular scope has no variables at all. If _no_ contexts in the
chain have non-nil variables, this is considered to mean that variables
are not allowed at all, which produces a different error message.
Indexing is pretty fundamental and it's also non-trivial, so having this
exposed will make it easier for it to be implemented consistently across
many different callers, including within calling applications.
Traversals are a generalized way to talk about paths taken from the scope
and from arbitrary values. These will be used for various analysis tasks,
such as determining what needs to be placed into a scope.
