lang: Add function values and lambdas

This adds a giant missing piece of the language: proper function values!
It is lovely to now understand why early programming language designers
didn't implement these, but a joy to now reap the benefits of them. In
adding these, many other changes had to be made to get them to "fit"
correctly. This improved the code and fixed a number of bugs.
Unfortunately this touched many areas of the code, and since I was
learning how to do all of this for the first time, I've squashed most of
my work into a single commit. Some more information:

* This adds over 70 new tests to verify the new functionality.

* Functions, global variables, and classes can all be implemented
natively in mcl and built into core packages.

* A new compiler step called "Ordering" was added. It is called by the
SetScope step, and determines statement ordering and shadowing
precedence formally. It helped remove at least one bug and provided the
additional analysis required to properly capture variables when
implementing function generators and closures.

* The type unification code was improved to handle the new cases.

* Light copying of Node's allowed our function graphs to be more optimal
and share common vertices and edges. For example, if two different
closures capture a variable $x, they'll both use the same copy when
running the function, since the compiler can prove if they're identical.

* Some areas still need improvements, but this is ready for mainstream
testing and use!

lang/util/util.go

@@ -38,3 +38,54 @@ func HasDuplicateTypes(typs []*types.Type) error {
 	}
 	return nil
 }
+
+// FnMatch is run to turn a polymorphic, undetermined list of functions, into a
+// specific statically typed version. It is usually run after Unify completes.
+// It returns the index of the matched function.
+func FnMatch(typ *types.Type, fns []*types.FuncValue) (int, error) {
+	// typ is the KindFunc signature we're trying to build...
+	if typ == nil {
+		return 0, fmt.Errorf("type of function must be specified")
+	}
+	if typ.Kind != types.KindFunc {
+		return 0, fmt.Errorf("type must be of kind Func")
+	}
+	if typ.Out == nil {
+		return 0, fmt.Errorf("return type of function must be specified")
+	}
+
+	// find typ in fns
+	for ix, f := range fns {
+		if f.T.HasVariant() {
+			continue // match these if no direct matches exist
+		}
+		// FIXME: can we replace this by the complex matcher down below?
+		if f.T.Cmp(typ) == nil {
+			return ix, nil // found match at this index
+		}
+	}
+
+	// match concrete type against our list that might contain a variant
+	var found bool
+	var index int
+	for ix, f := range fns {
+		_, err := typ.ComplexCmp(f.T)
+		if err != nil {
+			continue
+		}
+		if found { // already found one...
+			// TODO: we *could* check that the previous duplicate is
+			// equivalent, but in this case, it is really a bug that
+			// the function author had by allowing ambiguity in this
+			return 0, fmt.Errorf("duplicate match found for build type: %+v", typ)
+		}
+		found = true
+		index = ix // found match at this index
+	}
+	// ensure there's only one match...
+	if found {
+		return index, nil // w00t!
+	}
+
+	return 0, fmt.Errorf("unable to find a compatible function for type: %+v", typ)
+}