Handle substitutions in Booster directly

[geo2a]

Currently, in kore-rpc-booster we rely on Kore's simplifier to simplify the execution state before returning it to the client. One of the reasons for that is that Kore extracts the substitution from constraints and applies it to the configuration. We, however, could do it in Booster.

Here's an idea by @jberthold on what we should do:

I think we could take a different approach, namely to identify substitution-like constraints in the internal format and then apply the substitution as well as return it.

That would probably be part of the final simplification before returning the execute result. An even more invasive change would be to carry a substitution within the Pattern type which is accumulated as we rewrite the term (accommodates the substitution that is returned by kore-rpc when we fall back).
We do not have to extend the internal Pattern type with a substitution field if we only handle substitutions at the end.