Core and process dev

[kazupon]

The following functions are not yet implemented. (WIP)

• lev.abort
• lev.kill
• lev.exit

[hnakamur]

I fixed this just for tests to pass. Please see the last two commits in https://github.com/hnakamur/lev/commits/fix_core_and_process_dev/

We may need to change APIs as discussed between @kristate and @kazupon at IRC #lev-jp