Fix memory issues

- Use shared pointers for factory methods

I think the use of shared pointers in `symbolicallyExecute` is the
correct decision due to the filling of a map. I tried for a while to
make this a map of objects, but due to [object
slicing](https://en.wikipedia.org/wiki/Object_slicing) it would not be
possible to store a PredicatedValueBits in this map. (This would require
PredicatedValueBits to be redesigned into ValueBits class, but then the
internal methods get messy. Virtual functions was a clean answer to
this) Additionally, using `unique_ptr` proves difficult due to having to
store it in a map. And unique pointers cannot be copied.  As
`symbolicallyExecute` is the primary user of the factory methods, it
made sense for the factory methods to return a shared pointer instead of
a raw pointer or object and then wrapping this in a shared pointer
within symbolicallyExecute.

- Have XOR bits own their LHS/RHS

Having them as member objects instead of pointers I don't think makes
much sense due to them not being needed for most ValueBit objects, but
they would have to be initialized for all ValueBit instantiations.

- Use object values instead of pointers for bit representation

IMO the cleanest way. They're very lightweight anyway.

Author: joe-img