cleaned up usage of AST_BlockStatement

The following nodes were instances of AST_BlockStatement: AST_Scope,
AST_SwitchBlock, AST_SwitchBranch.  Also, AST_Try, AST_Catch, AST_Finally
were having a body instanceof AST_BlockStatement.

Overloading the meaning of AST_BlockStatement this way turned out to be a
mess; we now have an AST_Block class that is the base class for things
having a block of statements (might or might not be bracketed).  The
`this.body` of AST_Scope, AST_Try, AST_Catch, AST_Finally is now an array of
statements (as they inherit from AST_Block).

Avoiding calling superclass's _walk function in walkers (turns out we walked
a node multiple times).

tmp/test-node.js

@@ -41,7 +41,7 @@ time_it("compress", function(){
     ast = ast.squeeze(compressor);
 });
 
-var stream = UglifyJS.OutputStream({ beautify: false });
+var stream = UglifyJS.OutputStream({ beautify: true });
 time_it("generate", function(){
     ast.print(stream);
 });