Investigate failures in oj test suite

[eregon]

See https://github.com/ohler55/oj/pull/804/files
Some missing C API functions (like rb_ivar_foreach), some differences and some segfaults.

It might be useful to run this with managed Sulong regarding the segfaults to find the invalid memory accesses.

[aardvark179]

rb_ivar_foreach seems pretty simple, though it has some special cases in MRI for classes and modules. Implementing that allows most of those skipped specs to pass, but we get two failures and 1 error for trying to call __allocate__ on something which we don't support it on. I'll take a bit more of a look and create some issues for the problems.

[aardvark179]

PRs submitted to OJ so far

• Time fix ohler55/oj#810
• Range fix ohler55/oj#811
• Add some null checks to mark functions. ohler55/oj#812
• Create exceptions in a more platform independent way. ohler55/oj#813

[eregon]

Most issues with the oj gem should be fixed now with 01607d7.
We need to make a PR to oj to remove the now-unecessary skips.

[eregon]

I have a branch removing most skips: ohler55/oj@develop...eregon:oj:remove_most_truffleruby_skips
I'll fix rb_struct_s_members/rb_struct_members first though.

[eregon]

Most skips are removed with ohler55/oj#817.