Fully abstract VM

Some parts of the VM state are still concrete only. All data types should be an Expr instance, allowing for a completely symbolic execution environment.

For the case of gas, we can cheat a little and use a partially symbolic gas model (i.e. gas is either concrete, or unknown). This will allow us to support symbolic dynamic data without having to build a (slow & complex) symbolic gas model.

Start by tracing the VM state definitions and the handling of gas and Expr values. Identify which state data types remain concrete and review the existing symbolic execution tests, if any; done means all VM data types use Expr-compatible representations while gas remains either concrete or unknown.

Written by the indexing model from the issue text.