Data object encoding

[nevans]

This adds basic encoding/decoding for ruby 3.2 Data objects, using !ruby/data:ClassName (a mapping of member names to member values) and !ruby/data-with-ivars:ClassName tags (a mapping with members and ivars mappings). Like Marshal, this uses rb_struct_initialize to assign member values to new Data objects.

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Originally (in the first commit), I updated Visitor::ToRuby to simply call data.send(:initialize, **members). But #initialize freezes the object, so we can't set instance variables afterwards.

The second commit sets instance variables before calling #initialize, which I think will lead to unhappy surprises for some users. Unfortunately, Data doesn't give us any other mechanism for setting the members... except for the C API.

The third commit works around the above issue by copying how Marshal loads Data objects, delegating ToRuby#init_struct(obj, members) to rb_struct_initialize. I think this is the least surprising behavior for users. This also bypasses any user-defined #initialize method (which may be seen as a feature or a bug).

[nevans]

FYI: I haven't put any guards in yet to make this work for older versions of ruby (before Data was defined).

[nevans]

I'll update this to pass for earlier versions of ruby.

But I'm also looking for feedback on the approach. The more I think about it, the more I'm thinking this should use rb_struct_initialize from the C API, then copy over any ivars, then freeze. Unless I'm misreading the C code (very possible), that will skip any user-defined #initialize override, and just use the underlying Struct implementation... and, although that's not identical to rb_data_initialize_m, its close enough and it's exported and in the headers. That's what Marshal does, since Marshal just treats Data like Struct... which also means that Marshal does not freeze Data objects!

[nevans]

For what it's worth, I've updated the PR to:

1. pass with ruby versions down to 2.5 (at least on my machine)
2. use rb_struct_initialize from the C API

[nevans]

Sorry. I thought I had it passing before, but I must have made a mistake in my earlier tests with ruby 2.5 (perhaps I had somehow disabled warnings locally)? I've changed the guards from defined?(::Data.define) to RUBY_VERSION >= "3.1.0". Unfortunately, it's impossible to check if a method is defined without triggering deprecated constant warnings.

[nevans]

Any thoughts about this approach? (Is it too close to the 3.4.0 release to hope for getting support for Data in time for 3.4?)