Vendor dependencies for 0.3.0 release

vendor/rayon-core/build.rs

@@ -0,0 +1,7 @@
+// We need a build script to use `link = "rayon-core"`.  But we're not
+// *actually* linking to anything, just making sure that we're the only
+// rayon-core in use.
+fn main() {
+    // we don't need to rebuild for anything else
+    println!("cargo:rerun-if-changed=build.rs");
+}