Support multiple functions in Python frontend

[AlexanderViand-Intel]

Currently, the frontend primarily relies on a decorator that captures a single function.

@heir.compile()
def foo():
    # ....

There seem to be (at least) two ways to achieve multiple function support:

• have multiple decorated functions and use global state to collect all the functions
• use a ContextManager:

  with heir.compile(): 
     def foo():
        # ....
      
      def bar():
         # ....

I'm currently leaning towards the latter version, but since there's plenty of C++-side work that needs to be done first to fully support this, we have some time to decide.

[j2kun]

This implies the functions are calling each other, and I feel like we need to solve this in the compiler first before doing the frontend.