Report compile errors in 'using' declarations that aren't ever referenced.

Fixes #403

c++/src/capnp/compiler/compiler.c++

@@ -681,6 +681,11 @@ void Compiler::Node::traverse(uint eagerness, std::unordered_map<Node*, uint>& s
       for (auto& child: content->orderedNestedNodes) {
         child->traverse(eagerness, seen, finalLoader);
       }
+
+      // Also traverse `using` declarations.
+      for (auto& child: content->aliases) {
+        child.second->compile();
+      }
     }
   }
 }

c++/src/capnp/testdata/errors.capnp.nobuild

@@ -152,3 +152,6 @@ struct UseGeneric {
 
 const embedBadType :UInt32 = embed "binary";
 const embedNoSuchFile :Data = embed "no-such-file";
+
+using Baz = import "nosuchfile-unused.capnp".Baz;
+# Check that an import in an unused `using` still reports error.

c++/src/capnp/testdata/errors.txt

@@ -56,3 +56,4 @@ file:149:18-49: error: Double-application of generic parameters.
 file:150:38-43: error: Sorry, only pointer types can be used as generic parameters.
 file:153:30-44: error: Embeds can only be used when Text, Data, or a struct is expected.
 file:154:37-51: error: Couldn't read file for embed: no-such-file
+file:156:20-45: error: Import failed: nosuchfile-unused.capnp