Reapply #1295

[msimberg]

After reverting #1295 in #1334, this reapplies the original PR with a fix for let_value. The issue was accessing the wrong callable f inside set_value for let_value. I was still accessing the f from this, even though this had already been moved (and thus emptied) into a local variable. This also adds a unit test that triggers the issue before the fix.

bulk and when_all had similar issues which have also been fixed.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.01% (target: -1.00%)	✅ 95.59% (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (b207b3f)	18274	13810	75.57%
Head commit (c64991c)	18297 (+23)	13829 (+19)	75.58% (+0.01%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1337)	68	65	95.59%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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