Vendor dependencies

Let's see how I like this workflow.

vendor/anyhow/tests/test_ffi.rs

@@ -0,0 +1,18 @@
+#![deny(improper_ctypes, improper_ctypes_definitions)]
+
+use anyhow::anyhow;
+
+#[no_mangle]
+pub extern "C" fn anyhow1(err: anyhow::Error) {
+    println!("{:?}", err);
+}
+
+#[no_mangle]
+pub extern "C" fn anyhow2(err: &mut Option<anyhow::Error>) {
+    *err = Some(anyhow!("ffi error"));
+}
+
+#[no_mangle]
+pub extern "C" fn anyhow3() -> Option<anyhow::Error> {
+    Some(anyhow!("ffi error"))
+}