Fix struct layouts on GraalPy

[msimacek]

• The initial GraalPy support PR removed some struct fields with the intention of hiding them. But the fields are physically present, which caused layout mismatch when users tried to subclass them via PyO3. I add them back.
• I also exposed a few functions, which were previsously omitted on GraalPy, but are actually available.

CC @timfel

[davidhewitt]

Thanks for the PR! Should we be bumping our supported graalpy version, both by what we check for and what we test against in CI?

[msimacek]

Hi, there is no need to bump the checked version, I didn't introduce any dependency on anything from newer versions. I can update the version in the CI, but I'd prefer to do that in a separate PR.