fix deprecation

examples/fib.tengo

@@ -0,0 +1,8 @@
+fib := func(x) {
+	if x < 2 {
+		return x
+	}
+	return fib(x-1) + fib(x-2)
+}
+
+fib(35)

internal/builtins/readfile.go

@@ -1,9 +1,9 @@
 package builtins
 
 import (
-	"io/ioutil"
 	"monkey/internal/object"
 	"monkey/internal/typing"
+	"os"
 )
 
 // ReadFile ...
@@ -17,7 +17,7 @@ func ReadFile(args ...object.Object) object.Object {
 	}
 
 	filename := args[0].(*object.String).Value
-	data, err := ioutil.ReadFile(filename)
+	data, err := os.ReadFile(filename)
 	if err != nil {
 		return newError("IOError: error reading from file %s: %s", filename, err)
 	}

internal/builtins/writefile.go

@@ -1,9 +1,9 @@
 package builtins
 
 import (
-	"io/ioutil"
 	"monkey/internal/object"
 	"monkey/internal/typing"
+	"os"
 )
 
 // WriteFile ...
@@ -19,7 +19,7 @@ func WriteFile(args ...object.Object) object.Object {
 	filename := args[0].(*object.String).Value
 	data := []byte(args[1].(*object.String).Value)
 
-	err := ioutil.WriteFile(filename, data, 0755)
+	err := os.WriteFile(filename, data, 0755)
 	if err != nil {
 		return newError("IOError: error writing file %s: %s", filename, err)
 	}