Remove intern calls on FieldMapper#Names

remove internal callas on FieldMappers#Names, we properly reuse FieldMapper, so there is no need to try and call intern in order to reuse the names. This can be heavy with many fields and continuous mapping parsing.
Construction now loses the side effect of a `NullPointerException` when this class is misused by giving `null` values for everything except `sourcePath`, which could lead to new, unexpected `NullPointerException`s upon use.