add support of dictionary literal inferring
add to DictionaryType methods required to implement iteration over internal state - Pair that represents dict
add support of iteration calls on TypeDict to emulate how dictionary works
add support for the dict single iteration by explicit keys method call, iterates only once

The simplest case covered, when tier part of For operator is inferred to list of values. Only first value from the infered iterator used to infer for-body.
extend testing module with additional assert methods
add test for the dict literal and single-iteration for the FOR body infering