I added support for Trace and Span which are ULID by default.

For now, Span are not exported from ToMap(), Format(), Marshal() and LogValuer. Is this expected and how should we format it?

2 options come to my mind:

• add Span to each the stack trace and source fragments
• return a list of Span from the .Span() getter (not very useful since there is no context provided to each Span)

Currently, the .Span() getter only returns the Span of the parent oops.OopsError. The Trace is generated lazily by the getter only if none was provided.