RFR: Bring back support for GPR visitors

[Martin Junghanns]

On Wed, 14 Apr 2021 05:51:54 GMT, Sergey Ponomarev <github.com+415502+stokito 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.
>
> Marked as reviewed by stokito at github.com (no known OpenJDK username).

@stokito Thanks for the review! What are the next steps to get this merged / released?

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