don't unnecessarily register closures with the cycle collector

[zerbina]

Summary

Instead of all closures, only register those that contain potentially
cycle-introducing indirections as potential cycle roots, reducing
pressure on the cycle collector.

Details

While they don't need to be, the generated closure environment object
types were all marked as inheritable. Since the "potential cycle root"
detection treats non-final object types and object types containing
refs to non-final objects as potential cycle roots, this led to all
non-cyclic closure environments being candidates for being scanned by
the cycle collector.

Closure environment object types are now marked as final.

[zerbina]

/merge

[github-actions]

Merge requested by: @zerbina

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• split out from implement a new CGIR and C code generator #1625
• should further improve performance when working with closures (at least indirectly)