Vendor dependencies

Let's see how I like this workflow.

vendor/bitflags/src/example_generated.rs

@@ -0,0 +1,14 @@
+//! This module shows an example of code generated by the macro. **IT MUST NOT BE USED OUTSIDE THIS
+//! CRATE**.
+
+bitflags! {
+    /// This is the same `Flags` struct defined in the [crate level example](../index.html#example).
+    /// Note that this struct is just for documentation purposes only, it must not be used outside
+    /// this crate.
+    pub struct Flags: u32 {
+        const A = 0b00000001;
+        const B = 0b00000010;
+        const C = 0b00000100;
+        const ABC = Self::A.bits | Self::B.bits | Self::C.bits;
+    }
+}