[Grace-core] Multiple inheritance & module inheritance

[Michael Homer]

These are the examples motivating positional inheritance, also
available from <http://ecs.vuw.ac.nz/~mwh/positional-motivation.txt>.

Here is the problematic case with what Kim does now:
    class widget.new {
        makeWindow(self.width)
    }

    class button.new {
        inherits widget.new
        var width := 100
    }
(uninitialised variable error: width is used before set)
In reality, this may be a method using a field, rather than
the field itself, but to keep the example simple I have
left that out.

With position-dependent inheritance, we can resolve that so
downcalls are safe:
    class widget.new {
        makeWindow(self.width)
    }

    class button.new {
        var width := 100
        inherits widget.new
    }
(Works! width is now set before makeWindow(self.width) runs)

What if the style we're using requires upcalls, rather than
downcalls?  That will be a problem with inheritance at the
end:
    class widget.new {
        var textColour := "#000"
        method getColour(for) {
            if (for == "text") then {
                return textColour
            } else {
                return "#fff"
            }
        }
    }

    class button.new {
        var colour := getColour "text"
        inherits widget.new
    }
(Error! Same as the first example.)

But we can fix them by putting the inherits clause at the
top again:
    class widget.new {
        method getColour(for) { ... }
    }

    class button.new {
        inherits widget.new
        var colour := getColour "text"
    }
(Works! The superclass initialisation has run before use.)

What if we want both? We can order the operations so that
everything works out:
    class widget.new {
        makeWindow(self.width)
        method getColour(for) { ... }
    }

    class button.new {
        var width := 100
        inherits widget.new
        var colour := getColour "text"
    }
(Works! Required subclass initialisation runs before
inheritance that uses it, which itself runs before subclass
code that relies on superclass inheritance.)