Remove imports that are included in prelude

src/builtin.rs

 use std::collections::BTreeMap;
-use std::string::String;
-use std::vec::Vec;
 use std::fs::{self, File};
 use std::io::{stdout, Read, Write};
 use std::env;

src/input_editor.rs

-use std::string::String;
 use std::io::stdin;
 
 pub fn readln() -> Option<String> {

src/main.rs

 #![feature(box_syntax)]
 
 use std::collections::BTreeMap;
-use std::string::String;
-use std::vec::Vec;
-use std::boxed::Box;
 use std::fs::File;
 use std::io::{stdout, Read, Write};
 use std::env;