Create a Fact class for storing facts

Today, the congress runtime stores facts as Rules. This is inefficient from a memory perspective since each Rule contains a Literal, which in turn contains a Term for each column. Each of these is a python object, and each of them also contains a few extra fields like head, body, location, negated, etc. Using Rules is also inefficient with CPU since congress needs to construct all these objects. This blueprint proposes to use a Fact data structure to store each fact. A Fact is a subclass of a native tuple, plus one field for table name. This is much more efficient memorywise and CPUwise than using a Rule because there are no extra objects like Literal and Term. Preliminary tests show a 10x reduction in CPU for initializing tables plus a 3x reduction in memory use.