Vendor dependencies

Let's see how I like this workflow.

vendor/num-integer/build.rs

@@ -0,0 +1,13 @@
+extern crate autocfg;
+
+use std::env;
+
+fn main() {
+    // If the "i128" feature is explicity requested, don't bother probing for it.
+    // It will still cause a build error if that was set improperly.
+    if env::var_os("CARGO_FEATURE_I128").is_some() || autocfg::new().probe_type("i128") {
+        autocfg::emit("has_i128");
+    }
+
+    autocfg::rerun_path("build.rs");
+}