Provide a non-leaky API

[exhuma]

Currently, the package provides a "leaky abstraction" in the sense that calls to the main entry-point functions return data-types which are internal to puresnmp (most notably ObjectIdentifier).

The returned data-types should always be pure Python types.

Sometimes the "raw" data-types may provide useful functionalities, and as such a second entry-point should be provided to return the values unmodified. This would make it explicit to the end-user what kind of types are returned.