dead link tracking

[mockersf]

Currently dead link check is enabled but doesn't fail in CI due to a lot of dead links. See #1590

• Linked file at path struct.Span.html does not exist! will be fixed in tracing v0.2
• into_any* and as_any* comes from an issue in how downcast-rs is built/documented?
• sort_by_key should be solved by rust 1.52

[mnmaita]

I assume item number 3 is now resolved, but what about the issue with downcast-rs?