move lambda translation option from ATP to Sledgehammer, to avoid accidentally breaking Metis (its reconstruction code can only deal with combinators)

added option to control which lambda translation to use (for experiments)

fix subtle type inference bug in Metis -- different occurrences of the same Skolem might need to be typed differently, using paramify_vars overconstraints the typing problem

use monomorphic encoding as fallback, since they tend to produce fewer type errors

don't generate Waldmeister problems with only a conjecture, since it makes it crash sometimes

clearer unsound message

clarify fine soundness point

always unfold "Let"s is Sledgehammer, Metis, and MESON

unbreak Nitrox's parsing

merged