string: use "bl_pp_nonzero_vargs_count" for length hint
string: use "bl_pp_nonzero_vargs_count" for length hint

This commit uses "bl_pp_nonzero_vargs_count" for the length hint,
which is much more lightweight than "bl_pp_nonzero_vargs_count" and
works on Windows too as of now.

RafaGago/base_library,RafaGago/base_library,RafaGago/base_library