## Motivation
The `with_ancestry` methods on `NewSpan` and `ExpectedEvent` provide a
way to match whether the span or event is a contextual or explicit root
or if it has a contextual or explicit parent span.
However, in the case of matching on a contextual or explicit parent
span, only the span name could be used for matching. This is
sufficiently precise when testing tracing instrumentation in other
libraries or applications as opposed to testing tracing itself.
It is likely that a user would like to test that some span or event has
a specific span as a parent, and not just any span with a specific name,
in many cases, all the possible parent spans may have the same name.
This is the case when testing tracing instrumentation in Tokio.
## Solution
To solve this problem, the `Ancestry` struct was renamed to
`ExpectedAncestry` and in the case of expecting an explicit or
conextual parent, an `ExpectedSpan` object can be passed in. This
provides the maximum possible flexibility.
The convenience functions in the `expect` module now take
`Into<ExpectedSpan>` so that existing tests that pass a string type
object for the parent will see the same behaviour as previously and
shorthand use for expected Ids is also available.
Additionally, the span checking code has been unified between the
`MockCollector` and `MockSubscriber` cases and the assertion
descriptions have been improved to make them more readable.
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