Fixes #3824, fixes #19154, and hopefully #24094. Re-applies #23787.

[yglukhov]

The first commit reverts the revert of #23787.
The second fixes lambdalifting in convolutedly nested closures/closureiters. This is considered to be the reason of #24094, though I can't tell for sure, as I was not able to reproduce #24094 for complicated but irrelevant reasons. Therefore I ask @jmgomez, @metagn or anyone who could reproduce it to try it again with this PR.

I would suggest this PR to not be squashed if possible, as the history is already messy enough.

Some theory behind the lambdalifting fix:

• A closureiter that captures anything outside its body will always have :up in its env. This property is now used as a trigger to lift any proc that captures such a closureiter.
• Instantiating a closureiter involves filling out its :up, which was previously done incorrectly. The fixed algorithm is to use "current" env if it is the owner of the iter declaration, or traverse through :ups of env param until the common ancestor is found.

[metagn]

Looks good to my understanding, maybe @jmgomez can confirm the current langserver works as expected with this in case my reproduction isn't enough

[Araq]

Ooops, squashed the commits, sorry.

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 5fa96ef

Hint: mm: orc; opt: speed; options: -d:release
175169 lines; 8.294s; 655.312MiB peakmem

[ZoomRmc]

Yay, a negative LoC PR (not counting tests)!