add ring_char_0 class constraints to several simprocs (internal proofs of #n ~= 0 fail for type s not in this class); test simprocs using most general type classes instead of just int and rat.

simultaneous check; tight maxidx;

avoid separate typ_check phases, integrate into main term_check 0 instead (cf. its Syntax.check_typs in Type_Infer_Context.prepare);

clarified singleton_fixate: intersection with supersort is identity, only replace actual type inference parameters; tuned reject_other, after_infer_fixate;