Move Rust symbols required by C++ to a separate Buck target

Closes #168.
diff --git a/src/macros/mod.rs b/src/macros/mod.rs
new file mode 100644
index 0000000..d12d96b
--- /dev/null
+++ b/src/macros/mod.rs
@@ -0,0 +1,4 @@
+#[macro_use]
+mod assert;
+#[macro_use]
+mod concat;