Vendor dependencies

Let's see how I like this workflow.

vendor/hex/tests/version-number.rs

@@ -0,0 +1,11 @@
+#![allow(non_fmt_panic)]
+
+#[test]
+fn test_readme_deps() {
+    version_sync::assert_markdown_deps_updated!("README.md");
+}
+
+#[test]
+fn test_html_root_url() {
+    version_sync::assert_html_root_url_updated!("src/lib.rs");
+}