Fix using Definitions as arguments to Definitions

[jackkoenig]

Chisel creates new contexts whenever you're constructing a Definition. Previously, if you had a Definition in the parent context, the actual object would not be available in the child context (because it's a new context, Builder.components is cleared when you go to a new context). Now, we keep track of definitions explicitly including definitions from parent/outer scopes. This is implemented using a nested List for efficiency of constructing the datastructure while still allowing for efficient iteration.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
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Type of Improvement

• Performance improvement - but probably not measurable
• Bugfix

Desired Merge Strategy

• Squash

Release Notes

Fixes #3708

Reviewer Checklist (only modified by reviewer)

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• Did you do one of the following when ready to merge:
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[azidar]

Awesome! Good catch

[jackkoenig]

Awesome! Good catch

@tymcauley caught it, I just stared at the code until it fixed itself 🙂

[tymcauley]

Thanks so much for fixing this @jackkoenig!