Increase recursion_limit in librustc_plugin_impl.

To avoid rustdoc problems.

src/librustc_plugin_impl/lib.rs

@@ -8,6 +8,7 @@
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/")]
 #![feature(nll)]
+#![recursion_limit = "256"]
 
 use rustc_lint::LintStore;