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

misc tuning and simplification;

misc tuning;

tuned signature; tuned;

quickcheck invocations in mutabelle must not catch codegenerator errors internally

sort assignment before simultaneous term_check, not isolated parse_term; prefer Syntax.read_typ over Syntax.parse_typ, to include check phase for sort assignment; simplified Syntax_Phases.decode_sort/decode_typ; discontinued unused Proof_Context.check_tvar;