Use value-initialization in C++ to gracefully handle non-POD types

[jwollen]

Locals of types containing imported non-POD members currently get zeroed, leaving them in an invalid state. This replaces zeroing with value-initialization when in C++ mode.

[sinkingsugar]

Completely necessary for any serious interaction with c++ code like we are doing with NimTorch.

[sinkingsugar]

Nice, lately CI seems to be working nicely as intended! Nice catches.

[mratsim]

This would probably be necessary to support C++ destructors in seq status-im/nim-ttmath#10

Otherwise Clang (but not GCC) complains about:
error: flexible array member 'data' of type 'Foo []' with non-trivial destruction

[jwollen]

@mratsim I'll have a look at sequences/refs afterwards, but they might be more painful since much of them is implemented in the stdlib and not generic :/

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. If you think it is still a valid PR, please rebase it on the latest devel; otherwise it will be closed. Thank you for your contributions.

[timotheecour]

I think cppNonPod from #16750 is very related to this

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. If you think it is still a valid PR, please rebase it on the latest devel; otherwise it will be closed. Thank you for your contributions.