Fix and refactor decoder and translator

[sergeypospelov]

This PR fixes the problem with UModelBase ambiguity and refactors the whole decoding mechanism a little bit.

Details

The problem

We analyze function check:

int check(A a0) {
    int x = a0.x; // here we first check for NPE and get a case with an exception
    if (x == 0) {
        return 0;
    }
    return 1;
}

1. Suppose we do fast-check a0 == 0 in empty UModel and it has empty m0: KModel, so a0 evaluates to sample value null, because register reading was not translated yet.
2. Then we translate register reading into r0 := mkConst("r0") and for some reason m0(r0) equals to something different from null.
3. Then we resolve the exceptional case into JcTest and read r0 from the model m0, but now the model contains r0, we get something non-null, and that is the problem.

To fix that, we provide an expression translator in every component of UModelBase, so it first translates an expression and only then the expression is evaluated in a KModel.

Other

I decided to remove UCachingExprTranslator to simplify extension of our expressions and the total number of components. Now, UExprTranslator caches UExprs via KNonRecursiveTransformer and also caches initial values for URegionIds explicitly.

Lazy model components calls translate function, which takes an expression from a cache or explicitly transforms it.

Also fixed cache usage in UTreeMemoryUpdatesFolder visitor a little bit.