Extend generics example with more ADT polymorphism

[henesy]

The for { T => ; } statement can be used in an adt[T] as well.

From qrstuv on #inferno:

implement Dongmap;

include "sys.m";
	sys: Sys;
include "draw.m";

Dongmap: module {
	init:	fn(nil: ref Draw->Context, nil: list of string);
};

Map: adt[K, V]
for { K => hash: fn(nil: self K): int; } {    # ← This right here
	v:	V;
	add:	fn(nil: self ref Map[K, V], key: K, val: V);
	find:	fn(nil: self ref Map[K, V], key: K): V;
};

Wang: adt {
	hash:	fn(nil: self ref Wang): int;
};

WingDingDingaling: adt {
	sz:	int;
};

Map[K, V].add(m: self ref Map[K, V], key: K, val: V)
{
	m.v = val;
}

Map[K, V].find(m: self ref Map[K, V], key: K): V
{
	return m.v;
}

Wang.hash(nil: self ref Wang): int
{
	return 69105;
}

init(nil: ref Draw->Context, nil: list of string)
{
	sys = load Sys Sys->PATH;

	m := ref Map[ref Wang, ref WingDingDingaling](ref WingDingDingaling(2));
	m.add(ref Wang, ref WingDingDingaling(10));
	sys->print("%d\n", m.find(ref Wang).sz);
}