lang: types: Add nil type for placeholder

The nil type is being added, but only slightly. It is not meant for real
use in the language. It will not work in all situations, and it isn't
implemented for many methods.

It is being added only for being a "dummy" placeholder value in the
engine when we have an unused value being propagated along an edge.

lang/types/value.go

@@ -599,6 +599,50 @@ func (obj *Base) Func() interface{} {
 //	return nil
 //}
 
+// NilValue represents a nil value. It should be used only in rare situations.
+type NilValue struct {
+	Base
+}
+
+// NewNil creates a new nil value.
+func NewNil() *NilValue { return &NilValue{} }
+
+// String returns a visual representation of this value.
+func (obj *NilValue) String() string {
+	return "nil"
+}
+
+// Type returns the type data structure that represents this type.
+func (obj *NilValue) Type() *Type { return NewType("nil") }
+
+// Less compares to value and returns true if we're smaller. This panics if the
+// two types aren't the same. This always returns false for nil.
+func (obj *NilValue) Less(v Value) bool {
+	return false // they're the same
+}
+
+// Cmp returns an error if this value isn't the same as the arg passed in.
+func (obj *NilValue) Cmp(val Value) error {
+	if obj == nil || val == nil {
+		return fmt.Errorf("cannot cmp to nil")
+	}
+	if err := obj.Type().Cmp(val.Type()); err != nil {
+		return errwrap.Wrapf(err, "cannot cmp types")
+	}
+
+	return nil
+}
+
+// Copy returns a copy of this value.
+func (obj *NilValue) Copy() Value {
+	return &NilValue{}
+}
+
+// Value returns the raw value of this type.
+func (obj *NilValue) Value() interface{} {
+	return nil
+}
+
 // BoolValue represents a boolean value.
 type BoolValue struct {
 	Base