Update core/Process library for Windows.

[m-hilgendorf]

• Change clib/retrieve_process_state to accept a pointer to a Process instead of using pid.
• Remove usage of OpenProcess, as lookup by pid fails if called long after the process terminates.
• Return an error if retrieve_process_state fails on Windows (either in WaitForSingleObject or GetExitCodeProcess)
• Add a handle field to Process to hold the process handle created by CreateProcess.