go AST: First step towards augmenting AST with full type information.

- change ast.Ident back to contain the name and adjust all dependent code
- identifier object information will be added again through an optional
  typechecker phase (in the works).
- remove tracking of scopes in parser - it's easier to do this in a separate
  phase (in the works)
- in godoc, generate popup info table directly instead of through a formatter
  for simpler data flow (at the expense of a little bit more code)

Runs all tests.

As a result of this change, the currently shown popup information
(const, var, type, func, followed by identifier name) will not be
shown anymore temporarily.

R=rsc
CC=golang-dev
https://golang.org/cl/1994041

lib/godoc/source.html

@@ -7,11 +7,8 @@
 <script src="http://www.google.com/jsapi"></script>
 <script src="/doc/popups.js"></script>
 <script>
-var popup_data = [
-{.repeated section Data}
-	'{@|popupInfo}',
-{.end}
-]
+{# IdList is HTML-escaped by godoc}
+var popup_data = {IdList}
 
 google.load("jquery", "1");
 google.setOnLoadCallback(function() {.meta-left}
@@ -19,5 +16,5 @@ google.setOnLoadCallback(function() {.meta-left}
 {.meta-right});
 </script>
 
-{# Source is HTML-escaped elsewhere}
+{# Source is HTML-escaped by godoc}
 <pre>{Source}</pre>