rustdoc: Fix json output and input

Turns out a hash map with integer keys isn't serializable to json.

Closes #10115

src/test/run-make/rustdoc-json/Makefile

@@ -0,0 +1,4 @@
+-include ../tools.mk
+all:
+	$(RUSTDOC) -w json -o $(TMPDIR)/doc.json foo.rs
+	$(RUSTDOC) -o $(TMPDIR)/doc $(TMPDIR)/doc.json

src/test/run-make/rustdoc-json/foo.rs

@@ -0,0 +1,25 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[crate_id = "foo#0.1"];
+
+//! Very docs
+
+pub mod bar {
+
+    /// So correct
+    pub mod baz {
+        /// Much detail
+        pub fn baz() { }
+    }
+
+    /// *wow*
+    pub trait Doge { }
+}