Vendor dependencies

Let's see how I like this workflow.

vendor/bitflags/tests/compile-pass/redefinition/stringify.rs

@@ -0,0 +1,19 @@
+use bitflags::bitflags;
+
+// Checks for possible errors caused by overriding names used by `bitflags!` internally.
+
+#[allow(unused_macros)]
+macro_rules! stringify {
+    ($($t:tt)*) => { "..." };
+}
+
+bitflags! {
+    struct Test: u8 {
+        const A = 1;
+    }
+}
+
+fn main() {
+    // Just make sure we don't call the redefined `stringify` macro
+    assert_eq!(format!("{:?}", Test::A), "A");
+}