type checking and error handling for builtins improved.

object/str.go

@@ -1,11 +1,22 @@
 package object
 
-import "fmt"
+import (
+	"fmt"
+	"unicode/utf8"
+)
 
 type String struct {
 	Value string
 }
 
+func (s *String) Len() int {
+	return utf8.RuneCountInString(s.Value)
+}
+
+func (s *String) Bool() bool {
+	return s.Value != ""
+}
+
 func (s *String) Type() ObjectType {
 	return STRING_OBJ
 }