RFR: 7332: Show stacktraces for constant values from stacktrace pool

[Jean-Philippe Bempel]

Replace the default Object.ToString of JfrStackTraces by the top frame
and linked to the StacktraceView for better visualization

Example:
<img width="882" alt="Screenshot 2021-09-06 at 17 21 42" src="https://user-images.githubusercontent.com/4610701/132238400-702bfeeb-b368-4501-b022-72d4f326b461.png">

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Commit messages:
 - 7332: Show stacktraces for constant values from stacktrace pool

Changes: https://git.openjdk.java.net/jmc/pull/305/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=305&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7332
  Stats: 24 lines in 2 files changed: 21 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/305.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/305/head:pull/305

PR: https://git.openjdk.java.net/jmc/pull/305