In Python 2, objects automatically inherit the __hash__ of their parent
class. In Python 3, objects that override __eq__ do not automatically inherit
__hash__, so these objects were not hashable under Python 3. Additionally,
mutable BSON types and types that overide __eq__ but did not explicitly define
__hash__ had broken __hash__ methods under Python 2. This commit unifies the
hashing behavior between Python versions and fixes the __hash__ methods such
that two BSON objects hash the same only if they are equal.
N.B.: bson.code.Code and bson.regex.Regex are no longer hashable under Python 2
because they are mutable.
Add a 'compile_re' parameter to Collection.find, Collection.find_one, and
json_util.loads. If it's False, regular expressions are encoded as a new class,
Regex, instead of passed to re.compile(). This allows PyMongo to handle regular
expressions that don't compile in Python but are valid in other contexts like
MongoDB queries.
