Avoid needless concatenations of warning/error messages

[JaroslavTulach]

Pull Request Description

While investigating #182234656 I've noticed that GatherDiagnostics calls distinctBy and uses message string. However such string has to be composed and that takes time:

This PR eliminates the need for concatenation by associating each Diagnostic with an array of keys used to compute equals and hashCode.

Important Notes

Checklist

Please include the following checklist in your PR:

• [ x ] All code conforms to the Scala,
• All code has been tested:
  • [ x ] Unit tests have been written where possible.

sbt:runtime> testOnly *GatherDiagnosticsTest*
[info] compiling 3 Scala sources to /home/devel/NetBeansProjects/enso/enso.master/engine/runtime/target/scala-2.13/classes ...
[info] GatherDiagnosticsTest:
[info] Error Gathering
[info] - should work with expression flow (424 milliseconds)
[info] - should work with module flow (24 milliseconds)
[info] - should avoid duplication (1 second, 78 milliseconds)
[info] Run completed in 2 seconds, 731 milliseconds.
[info] Total number of tests run: 3
[info] Suites: completed 1, aborted 0
[info] Tests: succeeded 3, failed 0, canceled 0, ignored 0, pending 0
[info] All tests passed.

[kustosz]

Looks semantically dangerous if left as is.

[kustosz]

Now it's awesome :)

[hubertp]

Thanks, much easier to parse equals