A more general bsr/bsf implementation

[Johan Engelen via Digitalmars-d]

>> 2) Is the current sign-extend up to size_t's width really 
>> intended behavior?
>
> It's due to integer promotions, so it should only influence bsr 
> (when it is called with a signed type.)

Sorry for not being clear.
I understand why the current bsr behaves like it does, but what I 
meant is whether that is the desired behavior of bsr:
     bsr( byte(-1) ) == 31  (32-bit size_t)
     bsr( byte(-1) ) == 63  (64-bit size_t)
instead of
     bsr( byte(-1) ) == 7