Separation of concrete and symbolic execution

Disentangling concrete and symbolic representation and execution has a potential to significantly improve runtime performance while simultaneously making the codebase less complex and easier to maintain.

For example

- all the gas related computations could be removed from symbolic exploration
- concrete execution could use only concrete representation, without the need to deal with `Exprs`.

No files, tests, or entry points are named. Begin by mapping the concrete and symbolic representations and execution paths, then identify where gas computations and Exprs cross those paths. Done means the two execution modes are separated without changing their intended behavior and with performance or complexity improvements demonstrated.

Written by the indexing model from the issue text.