Use __del__ in Python to clean up Rust objects

[scooter-dangle]

__del__ more closely maps to what we want than context managers. For
instance, with the current code's use of context management, the only way to
make a collection of ZipCodeDatabases would be to explicitly nest
with ... as statements. As well, a context manager would complicate
persisting a Rust object that is expensive to initialize but cheap to use,
whereas __del__ supports this while still freeing the resources when the
reference count drops to zero.

There are two drawbacks to using __del__ over context management:

1. Memory cleanup in the presence of reference cycles in Python is less
   predictable.

• This is already an issue in Python that developers will need to be aware
  of. The fact that some of the objects involved will be cleaned up by
  calling a Rust function doesn't exacerbate that problem.

2. The Rust cleanup function might not run immediately upon the variable going
   out of scope

• Since the cleanup function is to avoid a memory leak, I don't see how this
  is any different than any other allocated object in Python. Additionally,
  we know in the Rust world to not rely on drop being called to guarantee
  properties like safety, so I think __del__ maps better to the concept of
  the drop function it will be calling.

Aside from __del__ reducing the restrictions on using Rust objects within
Python, it's also conceptually simpler. And to an audience that's already
learning Rust, it should be easier to teach in conjunction with teaching
Drop.

Fixes #60