KT-1859  filter method applied to a string does not iterate over chars 

Added collection like extension methods to String:
filter, filterTo, filterNot, filterNotTo, reverse, forEach, all, any, appendString, find, findNot, partition, flatMap, flatMapTo, fold, foldRight, reduce, reduceRight, groupBy, groupByTo, makeString, dropWhileTo, dropWhile, drop, takeWhileTo, takeWhile, take, toCollection, toLinkedList, toList, toCollection and toSet.