swap_pager: use iterators in swp_pager_meta_build

Add a method to use an iterator for pctrie insertion; this should
improve performance when the last search ended near the place where
the new item will be inserted.

Add an iterator argument to swp_pager_meta_build, so that the lookups
and insertions it does can be faster in the common case when keys are
bunched close together, or appear in sequence.

Reviewed by: alc
Differential Revision: https://reviews.freebsd.org/D46848