Remove Memory_stats library

[mrmr1993]

This PR removes the Memory_stats library, as part of the LowFillRate investigation.

This library was manually calling the garbage collector and dumping stats at regular intervals; previously we identified this as a source of slow-downs, but it appears that the code still remained in some places regardless.

Checklist:

• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them

[mrmr1993]

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[mrmr1993]

!ci-build-me

[mrmr1993]

!ci-build-me

[mrmr1993]

!ci-build-me

[mrmr1993]

!ci-build-me

[mrmr1993]

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