isoweek speedup

[tdhock]

TODO create performance test based on
Rdatatable/data.table#7144

[ANAMASGARD]

Sir @tdhock can I add the performance test (benchmark) that reproduces the isoweek() speedup scenario from data.table #7144 , as you asked for if you are not working on it .

[tdhock]

ok

[ANAMASGARD]

@tdhock Sir I have added the test now .