Permit child MastNodeIds to exceed the MastNodeIds of their parents

[PhilippGackstatter]

Describe your changes

Permit child MastNodeIds to exceed the MastNodeIds of their parents during deserialization. This came up in the context of #1534 (see this comment chain for details).

Deserialization seems to be the only place where we had a requirement on the mentioned property, judging by the uses of MastNodeId::from_u32_safe.

A new constructor for MastNodeId was introduced that is similar to the existing one, except that it checks against a total expected node count of the forest instead of the current node count of the forest. This allows child IDs to exceed their parent's IDs, such as in a forest like

[Join(1, 2), Block(foo), Block(bar)]

However, we still want forests like this to fail during deserialization:

[Block(foo), Join(0, 2)]

And the added constructor handles these scenarios and both are tested.

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.

[bobbinth]

I'm wondering if we should try the approach I described in #1534 (comment) before going with this direction.