Move ToString to collections::string

This also impls `FormatWriter` for `Vec<u8>`

src/test/run-pass/class-cast-to-trait-cross-crate-2.rs

@@ -11,7 +11,7 @@
 // aux-build:cci_class_cast.rs
 extern crate cci_class_cast;
 
-use std::to_string::ToString;
+use std::string::ToString;
 use cci_class_cast::kitty::cat;
 
 fn print_out(thing: Box<ToString>, expected: String) {

src/test/run-pass/send_str_treemap.rs

@@ -11,7 +11,7 @@
 extern crate collections;
 
 use std::str::{SendStr, Owned, Slice};
-use std::to_string::ToString;
+use std::string::ToString;
 use self::collections::TreeMap;
 use std::option::Some;