fix translation of higher-order equality ("fequal") if "precise_overloaded_args" is "true"

fix Vampire parsing problem

improve partially tagged encoding by adding a helper fact that coalesces consecutive "ti" tags

example tuning

remove at most one double negation -- any other double negations are part of some higher-order reasoning and should be left alone, cf. "HO_Reas.thy"

weaken the "expect" flag so that it doesn't trigger errors if a prover is not installed

added example to exercise higher-order reasoning with Sledgehammer and Metis

added Sledgehammer support for higher-order propositional reasoning