[lworld] RFR: 8341771: [lworld] Heap dump recognizing new flat fields formats

[Alex Menkov]

The fix re-implements flat object support in heap dumper.
New approach does not require changes in .hprof format.
Flat value objects are dumped as heap-allocated objects with generated ID.
For heap-allocated objects the ID is their address (oop), they are always aligned.
ID generator for flat objects uses unaligned values (so there is no conflicts with oops).

HeapDump test was reimplemented (it was disabled for a long time)
Changes in hprof test lib were reverted (they are not needed anymore)

testing: tier1..4,hs-tier5-svc

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Commit messages:
 - new implementation
 - revert old implementation
 - revert old implementation

Changes: https://git.openjdk.org/valhalla/pull/1700/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1700&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341771
  Stats: 1420 lines in 10 files changed: 297 ins; 941 del; 182 mod
  Patch: https://git.openjdk.org/valhalla/pull/1700.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1700/head:pull/1700

PR: https://git.openjdk.org/valhalla/pull/1700