Added fast radix order for numeric type

1) ordering numeric type is about 5-8x faster than "order(.)".
2) The sorting is automatically done as per data.table requirements - NaN < NA < numbers.
3) Added a R-wrapper "radixorder2" for easy call (not exported). `setkey()` is not yet modified (this should be done soon).
4) Tests added and tested successfully.
5) No NEWS updated (as it's internal change). Not sure if I should, yet.