Support multiple functions in Python frontend

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

```python
@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:
```python
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.