Inline cxx-symbols crate into cxx

The separation is no longer needed for Buck when linking with lld.
diff --git a/src/rust_vec.rs b/src/rust_vec.rs
index 5e7082a..6f1d567 100644
--- a/src/rust_vec.rs
+++ b/src/rust_vec.rs
@@ -3,7 +3,7 @@
 
 #[repr(C)]
 pub struct RustVec<T> {
-    repr: Vec<T>,
+    pub(crate) repr: Vec<T>,
 }
 
 impl<T> RustVec<T> {