Cleanup: NLL mutable stack building pattern

[nikomatsakis]

Follow-up cleanup from PR #246:

nll.rs has a clone-heavy pattern for building an immutable stack:

let stack = { let mut s = stack.clone(); s.push(this_entry.clone()); s }

This could be made more readable, possibly with a helper or a different data structure.

See: #246 (comment)

[nikomatsakis]

Note: I expect to tackle this as part of the NLL rewrite I'm working on.

[pritgit28]

Hello! I’m an Outreachy contributor.
May I be assigned this issue?

[tiif]

Hi @pritgit28, I think @nikomatsakis is planning to work on this. I opened a topic in zulip #t-types/formality > Good first issues for Outreachy, we can discuss what issue will be a good start there.

[pritgit28]

I'm sorry @tiif. Since it was a good first issue, I thought it was open to Outreachy contributors. I'm looking forward to finding an issue to work on there. Thank you