Fix errors on 1.7

[darsnack]

DataType no longer has a mutable field. The new function ismutabletype can be used instead. Similarly, abstract is now isabstracttype.

jl_new_datatype now takes in a fattrs argument which I think should be T.atomicfields (maybe @vtjnash can correct me). The change was added in JuliaLang/julia#37847

Only remaining error is that accessing the field N for Vararg{T} is now an undefined reference.

Fixes #107

[darsnack]

Okay all the errors should have been resolved @DhairyaLGandhi

[darsnack]

It seems we were only running into that issue for Vararg{T}, so I added a specific path for just that case and reverted the isdefined change. I also added some more tests for undefined references. There is one case that is currently broken (i.e. #43).

@vtjnash can you comment on what the fattrs argument to jl_new_datatype is supposed to represent? Seems like field attributes (e.g. atomic), but I wasn't sure what I should be passing in.

[vtjnash]

It is unlikely that you should be trying to access internal functions via ccall. The allocator for struct objects is an intrinsic function: Core._structtype(Main, :A, %3, %4, %5, false, 0) (see @Meta.lower struct A; end), though the other functions called there are also mandatory, so calling eval with a struct definition is perhaps the most reliable version.

[darsnack]

@CarloLucibello I don't have commit rights here, can you review?

The fixes here touch internals, but it is no more hacky than the rest of the package in that regard. A full overhaul to make the package more stable in the long term is needed (we already see failures on nightly), but not a task that I can complete in the near future. I would suggest this is merged so that BSON.jl is usable on 1.7.

[CarloLucibello]

old tests are passing, new tests are added, good enough for me

[darsnack]

We should tag a patch release too

[CarloLucibello]

done