make `fix` polymorphic in two type variables and add an exception enable version `fixt`
make `fix` polymorphic in two type variables and add an exception
enable version `fixt`

CodaFi/Swiftx,typelift/Swiftx,CodaFi/Swiftx,CodaFi/Swiftx,mpurland/Swiftx,typelift/Swiftx,mpurland/Swiftx,mpurland/Swiftx,typelift/Swiftx,sendyhalim/Swiftx,DanielAsher/Swiftx,pomu0325/Swiftx