Avoid assigning the MongoDB map/reduce function definitions to Python
variables that have the same name as the built-in Python functions map()
and reduce(), since that will effectively remove the possibility of
using those built-ins in surrounding code.
Don't needlessly retry to import _cbson each time the (public)
bson.has_c() function is called. The internal _use_c variable
that is set when the module is loaded already contains the