lang: types: Improve docs for New and initialize the list

If we use this to generate a zero value, we want to make sure it's
completely initialized in case we use it subsequently. We also improve
the docs at the same time.

lang/types/type.go

@@ -550,8 +550,12 @@ func NewType(s string) *Type {
 	return nil // error (this also matches the empty string as input)
 }
 
-// New creates a new Value of this type.
+// New creates a new Value of this type. It will represent the "zero" value. It
+// panics if you give it a malformed type.
 func (obj *Type) New() Value {
+	if obj == nil {
+		panic("malformed type")
+	}
 	switch obj.Kind {
 	case KindBool:
 		return NewBool()

lang/types/value.go

@@ -774,6 +774,7 @@ func NewList(t *Type) *ListValue {
 		return nil // sanity check
 	}
 	return &ListValue{
+		V: []Value{},
 		T: t,
 	}
 }