Tags on mappings and sequences

[ExpHP]

Currently, tags are only emitted on Scalar events, and not on MappingStart or SequenceStart. This in turn leads to a shortcoming in serde_yaml where tags cannot be used on array-like variants of an enum.

!vec3 [1.0, 0.5, 0.3]

Notice that libyaml supports tags on all three of these events.

This will be a breaking change, as it will require adding new fields to pub enum Event.

[kosta]

If you end up implementing this, please do it in a way that one can deserialize yaml that contains arbitrary tags and serialize it again with the same tags, without losing information.