Avoid unnecessary cloning in list_* methods

[Wukong247]

Currently, the list_* methods clone their return values even though there's no mutation happening. We can optimize memory usage by just returning references where possible.

[chaitika]

hi is this up for grabs, I would like to work on this

[mojoX911]

This is still up for grabs.

[a-khushal]

@mojoX911 can you please review #668

[mojoX911]

@a-khushal thanks for the PR. We will review it soon after the release (Dec end). It's not a high priority right now.