Refactor alloc/sync.rs into separate files

- `alloc::sync::Arc` now lives in `libraries/alloc/sync/arc.rs`
- `alloc::sync::Weak` now lives in `libraries/alloc/sync/weak.rs`
- tests now live in `alloc/tests/arc.rs`

Author: yoshuawuyts

library/alloc/src/sync.rs renamed to library/alloc/src/sync/arc.rs

@@ -0,0 +1,16 @@
+#![stable(feature = "rust1", since = "1.0.0")]
+
+//! Thread-safe reference-counting pointers.
+//!
+//! See the [`Arc<T>`][Arc] documentation for more details.
+
+mod arc;
+mod weak;
+
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use arc::Arc;
+
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use weak::Weak;
+
+use arc::ArcInner;