Segmentation fault is caused with pyext mode when a method takes an object of another class as input

Hi, thank you for maintaining this fantastic software.

I found that the following codon code compiled with `-pyext`:
```python
@dataclass(python=True, repr=True)
class Foo:
a: float

@dataclass(python=True, repr=True)
class Bar:
def func(self, foo: Foo):
print(foo)

def func(foo: Foo):
print(foo)
```
can cause segmentation fault by the following Python code
```python
from codon_module import Foo, Bar

Bar().func(Foo())
```
I also found the following code properly works:

```python
from codon_module import Foo, func

func(Foo())
```