I'd like to make the internal `jj` binary at Google log the exit code,
but `ExitCode` doesn't seem to provide any way of getting the numeric
code out of it. This patch therefore replaces `ExitCode` by `u8` and
lets the `main()` function convert it to `ExitCode`. It's a bit
unfortunate but I don't see a better solution. It doesn't seem worth
it to create our own newtype for it.
