* Closes#2365
* Implements the syntax `f@{x1 := def1; ...; xn := defn}` and `f@?{x1 :=
def1; ..; xn := defn}`. Each definition inside the `@{..}` is an
ordinary function definition. The `@?` version allows partial
application (not all explicit named arguments need to be provided). This
subsumes the old record creation syntax.
