Extended checking for %c conversion type in case the replacement is a str to make sure that str is of length 1. Made checker aware of mapping keys: errors are reported when specifiers with mapping keys are mixed with those without. Error is reported when mapping keys and stars (in precision of minimum field width) are mixed. When mapping keys are present, the replacement expression is not yet checked.
Added tests for checking of key mapping in string interpolation
Added checking of key mapping in string interpolation