RFR: 6617: Make return value its own element

[Jessye Coleman-Shapiro]

This patch addresses [JMC-6617](https://bugs.openjdk.java.net/browse/JMC-6617): Better probe format for return values.

Currently method return values in the agent are described using a parameter element with an index of -1. This patch makes a designated returnvalue element that can be used for return values.

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Commits:
 - 368cc573: Make return value its own element

Changes: https://git.openjdk.java.net/jmc/pull/6/files
 Webrev: https://webrevs.openjdk.java.net/jmc/6/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6617
  Stats: 272 lines in 11 files changed: 200 ins; 25 del; 47 mod
  Patch: https://git.openjdk.java.net/jmc/pull/6.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/6/head:pull/6

PR: https://git.openjdk.java.net/jmc/pull/6