Slottify expressions

[feliam]

• Added a metaclass magic to compile the __slots__ from parent classes automatically-ish. (xslots)
• Refactored Expression constructor arguments to be all kwargs based as suggested by the "Super Considered Super" thing.
• Added tests to check 100% of the Expressions are slotted. It uses __subclasses__ to ensure all are accounted for.
• ArrayProxy renamed to MutableArray
• Array's index_max index_bits value_bits -> length, index_size, value_size
• Added an ArrayConstant to hold tainted concrete "bytes" and other concrete arrays.
• Added a way to read the full model from a solved core so 2 consecutives solver.get_value wont solve the CS twice
• BitVec to Bitvec .... (tbc)