RFR: Bring back support for GPR visitors

[Martin Junghanns]

Hey @shipilev First of all: Thanks for this useful library. I wanted to switch from jol 0.10 to 0.15 in one of my projects because of some issues on openJDK 15. I discovered that the ability to register visitors had been removed at some point. The interface `org.openjdk.jol.info.GraphVisitor` is actually still around. I find that feature quite useful as it allows users to compute some custom statistics of the object graph. I tried to use the `GraphLayout` to access the GPRs, but it's all package-private.

It would be nice if we could bring the feature back. Also, if you think the approach is not ideal, we can maybe figure out something else.

Thanks for your time.

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Commit messages:
 - Bring back support for GPR visitors

Changes: https://git.openjdk.java.net/jol/pull/15/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jol&pr=15&range=00
  Stats: 22 lines in 4 files changed: 16 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jol/pull/15.diff
  Fetch: git fetch https://git.openjdk.java.net/jol pull/15/head:pull/15

PR: https://git.openjdk.java.net/jol/pull/15