As a major rewrite to the process/thread subsystem, this commits:
1. Implements threads as a first-class object, which represents a group of OS resources
and a thread of execution;
2. Implements processes as a first-class object that manages threads and maintains
the parent-child relationship between processes;
3. Refactors the code in process subsystem to follow the improved coding style and
conventions emerged in recent commits;
4. Refactors the code in other subsystems to use the new process/thread subsystem.
1. All generated, build files are now in a separate build directory;
2. The CLI tool supports three sub-commands: init, build, and run;
3. Refactor tests to use the new tool.
