Use generic function instances

Related to #1, it is currently not possible to use instances of generic functions:

```go

func GenericToString[T any](v T) string {
	return fmt.Sprint(v)
}

var mod = convgen.Module(
	convgen.ImportFunc(GenericToString[int]),
)
```

This compiles just fine, but will throw the error:
```
cannot use GenericToString[int] as function
```
