passing expressions as attribute names

[ostap]

accessing a field of an object (e.g. obj["field"]) can only be done using string literals (e.g. "field"). this enhancement should enable passing expressions as attribute names, (e.g. "field" ++ N). keep in mind that this change should not impact the performance, in other words obj["field"] should be evaluated at the same speed as it is today. this could be achieved by generating different instructions for the vm.