[jumbo] Add begin()/end() to Slice.

Waits for a current transaction to finish in TEST_CompactRange when the
`background_work_finished_signal_` signal may be emitted due to an error. This is a prospective fix for flaky crashes observed in b/351680196#comment5.

PiperOrigin-RevId: 663457574

This allows this type to meet the requirements of e.g.
std::ranges::range, which is necessary for it to work with the
std::span range constructor, or the "non-legacy" constructor for
Chromium's base::span.

Bug: none

Without this check decoding the footer in Table::Open() can read
uninitialized bytes from a buffer allocated on the stack if the file
was unexpectedly short.

In practice this is probably fine since this function validates a magic
number but MSan complains about branching on uninitialized data.

PiperOrigin-RevId: 525271012

PiperOrigin-RevId: 520556840

This PR temporarily removes a package that is currently broken on
GitHub's Ubuntu 22.04 installation. This is the most expedient way to
make the CI green again, so we can test any other changes we may want to
land.

PiperOrigin-RevId: 520206940