Abstract configuration cells to improve performance

Possibly related: https://github.com/runtimeverification/kontrol/pull/112

We are facing performance issues that are likely caused by the huge KEVM configurations that get generated in presence of multiple contracts.

As mentioned by @ehildenb, we might consider doing what Virgil implemented as a technique in Kasmer (which also gets quite huge configurations):

• When normal execution is going, it places a variable on the subconfiguration that gets huge (in our case, I guess the accounts cell, but we could also do fresh variables in the contents of each account in teh state, so that their storage isn't there, for example).
• When we get a branch, and can see that it's due to being too abstract, we concretize again using the stored abstraction, take the needed step of execution, then re-abstract.
  That way, the overall configuration does not get too big for the backend.

The issue names no files, tests, or entry points. Start by reviewing the related pull request #112 and the Kasmer approach mentioned in the description. Done means reducing oversized configurations while preserving the described concretization and re-abstraction behavior during branching.