Spans: assert_shape! ?

[o0Ignition0o]

Spans currently only allow us to serialize / deserialize so we can take snapshots.

However what we're mostly looking for at the moment, is making sure the shape remains.
Moreover we're losing snapshot capabilities as soon as some entries are non deterministic

an assert_shape! macro of some sorts would come in handy, we might be able to make sure that some Spans are children of others, and their names / metadata complies with our expectations.