Avoid potential corruption during unpack of static_variant

[abitmore]

Note: this is to redo #250 which was merged but reverted.

Before this change, when sv.set_which(w.value) is already done and the following sv.visit(...) throws, the staic_variant object passed into the function may be updated to something incomplete and may cause unintended problems.

Although usually we don't reuse static_variant variables, it's not guaranteed.

This does lead to additional data copy.

Reference: https://gitlab.syncad.com/hive/hive/-/merge_requests/1049

[sonarcloud]

Quality Gate passed

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0.0% Duplication on New Code

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