rustdoc: Show macros in documentation

Any macro tagged with #[macro_export] will be showed in the documentation for
that module. This also documents all the existing macros inside of std::macros.

Closes #3163
cc #5605
Closes #9954

src/librustdoc/doctree.rs

@@ -32,6 +32,7 @@ pub struct Module {
     impls: ~[Impl],
     foreigns: ~[ast::ForeignMod],
     view_items: ~[ast::ViewItem],
+    macros: ~[Macro],
 }
 
 impl Module {
@@ -52,6 +53,7 @@ impl Module {
             impls      : ~[],
             view_items : ~[],
             foreigns   : ~[],
+            macros     : ~[],
         }
     }
 }
@@ -157,6 +159,13 @@ pub struct Impl {
     id: ast::NodeId,
 }
 
+pub struct Macro {
+    name: Ident,
+    id: ast::NodeId,
+    attrs: ~[ast::Attribute],
+    where: Span,
+}
+
 pub fn struct_type_from_def(sd: &ast::StructDef) -> StructType {
     if sd.ctor_id.is_some() {
         // We are in a tuple-struct