feat: evaluation stack for detecting cyclic references

[cwaldren-ld]

Adds a data structure for detecting circular references during evaluation. It is called EvaluationStack and uses RAII to notice/forget keys.

Intended usage is like:

if (auto guard = stack.SeenPrerequisite("foo")) {
   // Now "foo" has been seen. Can't obtain another guard until this one is destroyed.
   assert_false(stack.SeenPrerequisite("foo"))
}