Problem: it's not possible to use newly written machinery
in statements like 'setVar' and 'setField'.

Solution: first of all we need to implement decomposition functionality:
when we assign a var new value, this variable might be decomposed,
so we need to decompose our value to and call 'setVar' recursively.
This functionality is implemented in 'Expr.Decompose'.
Secondly, before compile contract we need to fully decompose
storage into cells, to have at least one decomposed variable to
see the effect of implemented approach.