In particular, this allows restoring the terminal on crashes, which is
feasible now that we have the panic handler. Since std::process::exit() skips
destructors, we need to reshuffle some code. The "exit_without_destructors"
semantics (which std::process::exit() als has) was mostly necessary for C++
since Rust leaks global variables by default.
When fish crashes due to a panic, the terminal window is closed. Some
terminals keep the window around when the crash is due to a fatal signal,
but today we don't exit via fatal signal on panic.
There is the option to set «panic = "abort"» in Cargo.toml, which
would give us coredumps but also worse stacktraces on stderr.
More importantly it means that we don't unwind, so destructors are skipped
I don't think we want that because we should use destructors to
restore the terminal state.
On crash in interactive fish, read one more line before exiting, so the
stack trace is always visible.
In future, we should move this "read one line before exiting" logic to where
we call "panic!", so I can attach a debugger and see the stacktrace.
