Rework Sys.exit for eval

[Simn]

I've always hated how a Sys.exit(0) when running the interpreter would cause an error on the compilation server. This change addresses this by always throwing the Sys_exit exception, but making sure to properly catch it in compiler.ml. compile_ctx now returns the exit code, and HighLevel.entry keeps going (in case of --next) as long as the exit code is 0. Finally, it calls comm.exit, which does a legit exit in the stdio version and nothing in the pipe version (context error state is set by catch_completion_and_exit).