util/set: Add a helper to resize a set

util/set: Add a helper to resize a set

Often times you don't know how big a set will be and you want the code
to just grow it as needed.  However, sometimes you do know and you can
avoid a lot of rehashing if you just specify a size up-front.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>