Vendor dependencies

Let's see how I like this workflow.

vendor/syn/tests/common/mod.rs

@@ -0,0 +1,28 @@
+#![allow(dead_code)]
+#![allow(clippy::module_name_repetitions, clippy::shadow_unrelated)]
+
+use rayon::ThreadPoolBuilder;
+use std::env;
+
+pub mod eq;
+pub mod parse;
+
+/// Read the `ABORT_AFTER_FAILURE` environment variable, and parse it.
+pub fn abort_after() -> usize {
+    match env::var("ABORT_AFTER_FAILURE") {
+        Ok(s) => s.parse().expect("failed to parse ABORT_AFTER_FAILURE"),
+        Err(_) => usize::max_value(),
+    }
+}
+
+/// Configure Rayon threadpool.
+pub fn rayon_init() {
+    let stack_size = match env::var("RUST_MIN_STACK") {
+        Ok(s) => s.parse().expect("failed to parse RUST_MIN_STACK"),
+        Err(_) => 20 * 1024 * 1024,
+    };
+    ThreadPoolBuilder::new()
+        .stack_size(stack_size)
+        .build_global()
+        .unwrap();
+}