clearly label functions and basic blocks

[mnikander]

• introduce the mandatory main function
• update how the stack works, so it handles the main function, and the return from the main, somehow (or keep it as a separate command 'exit'?)
• ensure all basic blows are part of a function and can be clearly identified as such (perhaps via compound names such as @foo.entry)
• design decisions documented
• verify that the main function is always present
• check that IR code satisfies these properties