fix(core): resolveReferences should only update references that require updates on its second pass in nested stacks

[ShadowCat567]

Reason for this change

When we synthesize an app with nested stacks, we do two passes of resolveReferences which is an expensive operation that walks the construct tree and resolves all the tokens it finds.
We do not need to resolve tokens in every construct in the construct tree a second time after setting up our nested stack assets. We should only revisit the constructs that received nested stack assets and do a second pass resolve on them instead of doing a second pass resolve on the entire stack.

Description of changes

We have a new function resolveReferencesInElements which will resolve all references in a given construct. This is used by resolveReferences which passes it a DFS iterator which goes through all the constructs in the construct tree. This is also directly invoked on all of the nested stacks, instead of starting from the root of the construct tree (like we did previously).

Describe any new or updated permissions being added

N/A

Description of how you validated changes

Added new tests to ensure we are not missing any cross stack references and ensured current tests relating to nested stacks still pass.
When I tested on a stack with a lot of nested stacks I saw the time taken by the second resolve pass after defineNestedStackAsset drop from 3,404ms to 35ms.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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