Steps towards tracking scopes for identifiers.

- Identifiers refer now to the language entity (Object)
	  that they denote. At the moment this is at best an
	  approximation.

	- Initial data structures for language entities (Objects)
          and expression types (Type) independent of the actual
	  type notations.

	- Initial support for declaring and looking up identifiers.

	- Updated various dependent files and added support functions.

	- Extensively tested to avoid breakage. This is an AST change.

R=rsc
CC=golang-dev, rog
https://golang.org/cl/189080

src/cmd/godoc/snippet.go

@@ -36,7 +36,7 @@ func (s *snippetStyler) LineTag(line int) (text []uint8, tag printer.HTMLTag) {
 
 
 func (s *snippetStyler) Ident(id *ast.Ident) (text []byte, tag printer.HTMLTag) {
-	text = strings.Bytes(id.Value)
+	text = strings.Bytes(id.Name())
 	if s.highlight == id {
 		tag = printer.HTMLTag{"<span class=highlight>", "</span>"}
 	}
@@ -115,7 +115,7 @@ func NewSnippet(decl ast.Decl, id *ast.Ident) (s *Snippet) {
 	if s == nil {
 		s = &Snippet{
 			id.Pos().Line,
-			fmt.Sprintf(`could not generate a snippet for <span class="highlight">%s</span>`, id.Value),
+			fmt.Sprintf(`could not generate a snippet for <span class="highlight">%s</span>`, id.Name()),
 		}
 	}
 	return