ENH: prevent conversion of string to regExp in stringListOps (closes #739)

* For most cases, this conversion would be largely unintentional
  and also less efficient. If the regex is desirable, the caller
  should invoke it explicitly.
  For example,

      findStrings(regExp(str), listOfStrings);

  Or use one of the keyType, wordRe, wordRes variants instead.
  If string is to be used as a plain (non-regex) matcher,
  this can be directly invoked

      findMatchingStrings(str, listOfStrings);

  or using the ListOps instead:

      findIndices(listOfStrings, str);

* provide function interfaces for keyType.