RFR: 8184166: SortedList does not free up memory

[Cormac Redmond]

Fix SortedList memory leak by clearing unused entries in the internal array when the source list shrinks. Also nullify tempElement.e after use,

Adding "--add-opens=javafx.base/javafx.collections.transformation=ALL-UNNAMED" to allow reflection access from tests.

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Commit messages:
 - 8184166: fixing duplicated comparator
 - 8184166: Adding a test, and improving readability of existing
 - 8184166: Updating SortedListTest's testNonRequiredInternalReferencesAreNull() to include additional comparators and combinations
 - Merge remote-tracking branch 'credmond/fixes/sortablelist-gc' into fixes/sortablelist-gc
 - 8184166: SortedList: nullify out-of-range elements and ensure tempElement is cleared, minor optimisations to aid GC
 - 8184166: SortedList: nullify out-of-range elements and ensure tempElement is cleared, minor optimisations to aid GC

Changes: https://git.openjdk.org/jfx/pull/2000/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2000&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8184166
  Stats: 163 lines in 3 files changed: 162 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/2000.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2000/head:pull/2000

PR: https://git.openjdk.org/jfx/pull/2000