Consistently use set.is_empty and set.is_non_empty.

compiler/*.m:
deep_profiler/*.m:
    As above.

library/set.m:
library/set_bbbtree.m:
library/set_ctree234.m:
library/set_ordlist.m:
library/set_tree234.m:
library/set_unordlist.m:
    Add is_non_empty to the set modules that did not already have it.
    (Some did, some didn't.)

    Make the documentation of empty, is_empty, non_empty and is_non_empty
    consistent.