This is the last use of Read/WriteBytesExt. The byteorder crate is great, but
we don't need an abstraction of endianness. Let's simply use the std functions.
Allows automatic recovery when encountering stale lockfiles, and more
efficient blocking rather than polling for fresh ones. The previous
implementation is preserved for other platforms.
Let's acknowledge everyone's contributions by replacing "Google LLC"
in the copyright header by "The Jujutsu Authors". If I understand
correctly, it won't have any legal effect, but maybe it still helps
reduce concerns from contributors (though I haven't heard any
concerns).
Google employees can read about Google's policy at
go/releasing/contributions#copyright.
The `testutils` module should ideally not be part of the library
dependencies. Since they're used by the integration tests (and the CLI
tests), we need to move them to a separate crate to achieve that.
