collections: Implement String::drain(range) according to RFC 574

`.drain(range)` is unstable and under feature(collections_drain).

This adds a safe way to remove any range of a String as efficiently as
possible.

As noted in the code, this drain iterator has none of the memory safety
issues of the vector version.

RFC tracking issue is #23055

src/test/run-pass/sync-send-iterators-in-libcollections.rs

@@ -21,6 +21,7 @@ use collections::{BitSet, BitVec};
 use collections::{BTreeMap, BTreeSet};
 use collections::EnumSet;
 use collections::LinkedList;
+use collections::String;
 use collections::Vec;
 use collections::VecDeque;
 use collections::VecMap;
@@ -99,4 +100,5 @@ fn main() {
 
     all_sync_send!(Vec::<usize>::new(), into_iter);
     is_sync_send!(Vec::<usize>::new(), drain(..));
+    is_sync_send!(String::new(), drain(..));
 }