Generalize/improve parsing of pointer declarations.

Substantial improvements and generalizations to the parsing and
declaration handling code to properly parse declarations involving
pointers.  (No change to user-visible functionality, but this
lays groundwork for supporting a more general pointer model.)

ast.cpp

@@ -51,6 +51,8 @@ ASTNode::~ASTNode() {
 
 void
 AST::AddFunction(Symbol *sym, const std::vector<Symbol *> &args, Stmt *code) {
+    if (sym == NULL)
+        return;
     functions.push_back(new Function(sym, args, code));
 }