further lower penalty associated with existentials in Sledgehammer's relevance filter, so that "exhaust" facts are picked up

reenabled equality proxy in Metis for higher-order reasoning

added "SMT." qualifier for constant to make it possible to reload "smt_monomorph.ML" from outside the "SMT" theory (for experiments) -- this is also consistent with the other SMT constants mentioned in this source file

reflect option renaming in doc + do not document the type systems poly_preds? and poly_tags?, since they are virtually identical to the non-? versions

unfold set constants in Sledgehammer/ATP as well if Metis does it too

do not pollute relevance filter facts with too many facts about the boring set constants Collect and mem_def, which we might anyway unfold depending on Meson's settings

renamed "max_mono_instances" to "max_new_mono_instances" and changed its semantics accordingly