all: support interface asserts in interp

This adds support for the math/rand package.

testdata/stdlib.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"math/rand"
 	"os"
 )
 
@@ -9,4 +10,6 @@ func main() {
 	fmt.Println("stdin: ", os.Stdin.Fd())
 	fmt.Println("stdout:", os.Stdout.Fd())
 	fmt.Println("stderr:", os.Stderr.Fd())
+
+	fmt.Println("pseudorandom number:", rand.Int31())
 }