Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/src/macros/assert.rs

@@ -0,0 +1,7 @@
+#[macro_export]
+#[doc(hidden)]
+macro_rules! const_assert_eq {
+    ($left:expr, $right:expr $(,)?) => {
+        const _: [(); $left] = [(); $right];
+    };
+}

vendor/cxx/src/macros/concat.rs

@@ -0,0 +1,8 @@
+#[macro_export]
+#[doc(hidden)]
+macro_rules! attr {
+    (#[$name:ident = $value:expr] $($rest:tt)*) => {
+        #[$name = $value]
+        $($rest)*
+    };
+}

vendor/cxx/src/macros/mod.rs

@@ -0,0 +1,4 @@
+#[macro_use]
+mod assert;
+#[macro_use]
+mod concat;