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/funcs/maplookup_polyfunc.go

@@ -30,6 +30,10 @@ const (
 	// starts with an underscore so that it cannot be used from the lexer.
 	// XXX: change to _maplookup and add syntax in the lexer/parser
 	MapLookupFuncName = "maplookup"
+
+	argNameMap = "map"
+	argNameKey = "key"
+	argNameDef = "default"
 )
 
 func init() {
@@ -48,6 +52,15 @@ type MapLookupPolyFunc struct {
 	closeChan chan struct{}
 }
 
+// ArgGen returns the Nth arg name for this function.
+func (obj *MapLookupPolyFunc) ArgGen(index int) (string, error) {
+	seq := []string{argNameMap, argNameKey, argNameDef}
+	if l := len(seq); index >= l {
+		return "", fmt.Errorf("index %d exceeds arg length of %d", index, l)
+	}
+	return seq[index], nil
+}
+
 // Polymorphisms returns the list of possible function signatures available for
 // this static polymorphic function. It relies on type and value hints to limit
 // the number of returned possibilities.
@@ -116,20 +129,20 @@ func (obj *MapLookupPolyFunc) Polymorphisms(partialType *types.Type, partialValu
 	typFunc := &types.Type{
 		Kind: types.KindFunc, // function type
 		Map:  make(map[string]*types.Type),
-		Ord:  []string{"map", "key", "default"},
+		Ord:  []string{argNameMap, argNameKey, argNameDef},
 		Out:  nil,
 	}
-	typFunc.Map["map"] = typ
-	typFunc.Map["key"] = typ.Key
-	typFunc.Map["default"] = typ.Val
+	typFunc.Map[argNameMap] = typ
+	typFunc.Map[argNameKey] = typ.Key
+	typFunc.Map[argNameDef] = typ.Val
 	typFunc.Out = typ.Val
 
 	// TODO: don't include partial internal func map's for now, allow in future?
 	if typ.Key == nil || typ.Val == nil {
 		typFunc.Map = make(map[string]*types.Type) // erase partial
-		typFunc.Map["map"] = types.TypeVariant
-		typFunc.Map["key"] = types.TypeVariant
-		typFunc.Map["default"] = types.TypeVariant
+		typFunc.Map[argNameMap] = types.TypeVariant
+		typFunc.Map[argNameKey] = types.TypeVariant
+		typFunc.Map[argNameDef] = types.TypeVariant
 	}
 	if typ.Val == nil {
 		typFunc.Out = types.TypeVariant
@@ -211,7 +224,13 @@ func (obj *MapLookupPolyFunc) Validate() error {
 // Info returns some static info about itself. Build must be called before this
 // will return correct data.
 func (obj *MapLookupPolyFunc) Info() *interfaces.Info {
-	typ := types.NewType(fmt.Sprintf("func(map %s, key %s, default %s) %s", obj.Type.String(), obj.Type.Key.String(), obj.Type.Val.String(), obj.Type.Val.String()))
+	var typ *types.Type
+	if obj.Type != nil { // don't panic if called speculatively
+		// TODO: can obj.Type.Key or obj.Type.Val be nil (a partial) ?
+		k := obj.Type.Key.String()
+		v := obj.Type.Val.String()
+		typ = types.NewType(fmt.Sprintf("func(map %s, key %s, default %s) %s", obj.Type.String(), k, v, v))
+	}
 	return &interfaces.Info{
 		Pure: true,
 		Memo: false,
@@ -245,9 +264,9 @@ func (obj *MapLookupPolyFunc) Stream() error {
 			}
 			obj.last = input // store for next
 
-			m := (input.Struct()["map"]).(*types.MapValue)
-			key := input.Struct()["key"]
-			def := input.Struct()["default"]
+			m := (input.Struct()[argNameMap]).(*types.MapValue)
+			key := input.Struct()[argNameKey]
+			def := input.Struct()[argNameDef]
 
 			var result types.Value
 			val, exists := m.Lookup(key)