RFR: CODETOOLS-7902930: JOL: Bring back GraphWalker visitors [v2]

[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.

Martin Junghanns has updated the pull request incrementally with one additional commit since the last revision:

  Avoid virtual calls in GraphWalker#walk

-------------

Changes:
  - all: https://git.openjdk.java.net/jol/pull/15/files
  - new: https://git.openjdk.java.net/jol/pull/15/files/0d75043b..c47ac838

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jol&pr=15&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jol&pr=15&range=00-01

  Stats: 25 lines in 1 file changed: 5 ins; 14 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