Hide C header implementation details related to actor pad size.

[jemc]

Previously, in the pony.h header that exposes the "public API" of the Pony runtime, information about the size of an actor struct was published as public information.

This change removes that from the public header into a private header, to hide
implementation details that may be subject to change in future versions of the Pony runtime.

This change does not impact Pony programs - only C programs using the pony.h header to use the Pony runtime in some other way than inside of a Pony program, or to create custom actors written in C.

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This commit moves the PONY_ACTOR_PAD_SIZE preprocessor value and pony_actor_pad_t struct into actor.h.

[ponylang-main]

Hi @jemc,

The changelog - changed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3960.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

why kill off the struct? you are "using it" now in two different locations. seems reasonable to move this into actor.h and then still use it as it was before in cycle.c and actor.c.

[jemc]

I plan to kill both uses of it in a future PR. Having this type exist whose memory layout is dependent on that pad size is antithetical with the goal we discussed of making the pad size be variable based on configuration settings at LLVM link/optimization time.

If you want me to, I can revert that part of the change in this PR, but even if so, I'm just going to remove it in a future PR.

What is your preference about how to proceed?

[SeanTAllen]

Leave now remove later. Feels more contained and easier to understand.

[jemc]

@SeanTAllen - The PR has been updated accordingly.