Pure/term/fastype_of1: renamed from fastype_of
Pure/term/fastype_of: new, takes only one argument (like type_of)
Pure/Syntax/printer/is_prop: now calls fastype_of1
Pure/pattern: now calls new fastype_of in three places
Pure/logic/mk_equals,mk_flexpair: now calls fastype_of instead of type_of.
So it no longer checks t properly -- but it never checked u anyway, and all
existing calls are derived from certified terms...