Add proper method to access scope root object

[jaredoconnell]

Changes introduced with this PR

This method adds validation for accessing the scope's root, and can be used outside of the SDK. It validates three possible mistakes.
There are three potential places to use this in the engine that I will need to address later.

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[dbutenhof]

The validations are a nice improvement over the direct dereference; although it "probably" gets called in other test cases where failure would be detected, an explicit "pass" test case would be nice.

[webbnh]

Looks generally good, but I don't think we should be panicking as a result of bad user input...and I have a couple of other comments.

[mfleader]

case_mismatched_root:
  root: wrong
  objects:
    a:
      id: a

case_mismatched_root_id:
  root: a
  objects:
    a:
      id: wrong

case3:
  root: a
  objects:
    wrong:
      id: a

The first two cases are what I believe your unit tests are addressing. Is case3 more so an issue with Object construction than Scope construction?

[jaredoconnell]

case_mismatched_root:
  root: wrong
  objects:
    a:
      id: a

case_mismatched_root_id:
  root: a
  objects:
    a:
      id: wrong

case3:
  root: a
  objects:
    wrong:
      id: a

The first two cases are what I believe your unit tests are addressing. Is case3 more so an issue with Object construction than Scope construction?

The last one is also a scope problem, because "wrong" is a key in the scope map. It doesn't match the object ID of "a".

[jaredoconnell]

I had written this earlier, but couldn't post it because of a Github error.