refactor Method definition to make space for macros

This change propagates to many locations, but because of the
Macro Exterminator (or, more properly, the invariant that it
protects), macro invocations can't occur downstream of expansion.
This means that in librustc and librustdoc, extracting the
desired field can simply assume that it can't be a macro
invocation. Functions in ast_util abstract over this check.

src/libsyntax/print/pprust.rs

@@ -245,6 +245,10 @@ pub fn arg_to_string(arg: &ast::Arg) -> String {
     to_string(|s| s.print_arg(arg))
 }
 
+pub fn mac_to_string(arg: &ast::Mac) -> String {
+    to_string(|s| s.print_mac(arg))
+}
+
 pub fn visibility_qualified(vis: ast::Visibility, s: &str) -> String {
     match vis {
         ast::Public => format!("pub {}", s),
@@ -342,6 +346,7 @@ impl<'a> State<'a> {
         match self.s.last_token() { pp::End => true, _ => false }
     }
 
+    // is this the beginning of a line?
     pub fn is_bol(&mut self) -> bool {
         self.s.last_token().is_eof() || self.s.last_token().is_hardbreak_tok()
     }
@@ -627,6 +632,7 @@ impl<'a> State<'a> {
         }
     }
 
+    /// Pretty-print an item
     pub fn print_item(&mut self, item: &ast::Item) -> IoResult<()> {
         try!(self.hardbreak_if_not_bol());
         try!(self.maybe_print_comment(item.span.lo));
@@ -998,11 +1004,26 @@ impl<'a> State<'a> {
         try!(self.hardbreak_if_not_bol());
         try!(self.maybe_print_comment(meth.span.lo));
         try!(self.print_outer_attributes(meth.attrs.as_slice()));
-        try!(self.print_fn(&*meth.decl, Some(meth.fn_style), abi::Rust,
-                        meth.ident, &meth.generics, Some(meth.explicit_self.node),
-                        meth.vis));
-        try!(word(&mut self.s, " "));
-        self.print_block_with_attrs(&*meth.body, meth.attrs.as_slice())
+        match meth.node {
+            ast::MethDecl(ident, ref generics, ref explicit_self, fn_style, decl, body, vis) => {
+                try!(self.print_fn(&*decl, Some(fn_style), abi::Rust,
+                                   ident, generics, Some(explicit_self.node),
+                                   vis));
+                try!(word(&mut self.s, " "));
+                self.print_block_with_attrs(&*body, meth.attrs.as_slice())
+            },
+            ast::MethMac(codemap::Spanned { node: ast::MacInvocTT(ref pth, ref tts, _),
+                                            ..}) => {
+                // code copied from ItemMac:
+                try!(self.print_path(pth, false));
+                try!(word(&mut self.s, "! "));
+                try!(self.cbox(indent_unit));
+                try!(self.popen());
+                try!(self.print_tts(tts.as_slice()));
+                try!(self.pclose());
+                self.end()
+            }
+        }
     }
 
     pub fn print_outer_attributes(&mut self,