Make impAnn a bit more useful, by making the logs scoped by impAnn

[lehins]

Description

Reviewing #3852 made me realize that sometimes we want to display lengthy Imp logs only when there is some failure in the current scope, but not otherwise.

So, imagine we have some very lengthy log entry (in this example: "Found a problem with x and y"), but we want to show this only when x and y are not equal, but for all ther failures in the test this log entry would not be relevant and should not be displayed. Then instead of writing this:

when (x /= y) $ do
  logEntry "Found a problem with x and y"
  expectationFailure $ showTreeDiff x y

we can now write this:

impAnn "Ensuring x and y validity" $ do
  logEntry "Found a problem with x and y"
  x `shouldBeExpr` y

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[teodanciu]

Just a question, approving (since I'm almost certain of the answer, but just to check that I understand).
Looks good to me

[Soupstraw]

Nice!