XS tweaks

in sieve_primes stop using the Perl stack ASAP to free up registers/C stack
space, NULL assignment is optimized away if SP is never used again

in trial_factor group the functions into 1 and 2 arg groups for a future
optimization

in _vcallsubn, since SP is used only once, don't waste memory keeping
it around long before first used
Merge pull request #8 from bulk88/master

XS tweaks