This commit makes the reachability tracker directly return changes to
implications from `propagate_all`, rather than stashing them and
expecting callers to access them through a separate `pushed` method.
This API changes allows `propagate_all` to assume that the caller will
consume the changes before the next call to `propagate_all`, which in
turn guarantees that `pushed_changes` only contains changes from the
last `propagate_all` call, rather than possibly changes from previous
calls as well.

This commit changes the shape of `TrackerEvent` to include the
source/target distinction in a `Location` rather than separate enum
variants. Apart from less boilerplate, this speeds up the frontier
logging in `propagate_all` as we can directly use the contents of
`pushed_changes`, instead of having to split them into target and source
changes.

A side effect of this is that the definition of
`ProgressEventTimestampVec` changes, which also affects the
`TimelyProgressEvent` type.