RFR: Bring back support for GPR visitors

[Aleksey Shipilev]

On Mon, 12 Apr 2021 16:00:49 GMT, Martin Junghanns <github.com+318788+s1ck at openjdk.org> wrote:

> 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.

Sorry this got neglected. The underlying reason to remove GPRs was to simplify (and thus optimize) the walkers. I'll try to arrange time to see what can be done here without sacrificing that again.

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