refactor(library/tactic): rename placeholder_elaborator to class_instance_synth
feat(library/tactic/class_instance_synth): add mk_class_instance procedures
fix(hott/init): minimize number of universe parameters
refactor(hott/init/util.hlean): modify definition to make it more convenient for definitional package
feat(library/util): add more auxiliary procedures
feat(library/tactic/class_instance_synth): add mk_hset_instance procedure