use context manager instead of global variable

Proposed usage:

```python
x = 5

with switch(x) as sw:

 if sw.case(4):
 assert False

 if sw.case(5):
 assert True
 sw.fallthrough()

 if sw.case(lambda x: x > 3):
 assert True

 if sw.case(6):
 assert False

 if sw.default():
 assert False
```