Reject indirect incompatible updates to composite declarations

[SupunS]

Closes #771

Description

Currently, doing incompatible changes to a composite type (like changing field types, adding new fields, etc) is not allowed. However, it is possible to remove an existing composite type, and add another composite type with the same name, but with a different structure, which has the same effect as doing an incompatible change.

This PR rejects such indirect changes to a struct. With this,

• Removing any existing composite type declaration is not allowed
• Adding new composite type declarations is allowed

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[turbolent]

This is great! Good catch 👍