Data object encoding

[nevans]

The first commit adds basic encoding/decoding for ruby 3.2 Data objects in (what seems to me) the most straightforward way: Visitor::YamlTree copies all of the members out into a mapping, and Visitor::ToRuby calls #initialize with that mapping. This could lead to side effects, if users have overridden #initialize in weird ways, but that's true of Struct and the attr writer methods and many of the types in Visitors::ToRuby, too.

The second commit handles instance variables, and I feel less happy with that implementation. It sets the ivars before calling initialize, which feels wrong. But Data doesn't give us any mechanism for setting the members other than 1) initialize, or 2) drop down into the C API. Since initialize freezes the object, we need to set the ivars before that. I think this is a reasonable compromise—if users need better handling, they can implement their own encode_with and init_with. But it will lead to unhappy surprises for some users.

Alternatively, maybe we could use the C API, similarly to Marshal. Psych is already using the C API for path2class and build_exception. This would be the least surprising behavior for users, I think.

Edited to add: I added ToRuby#init_struct(obj, members) to the C extension, which delegates to rb_struct_initialize.

[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