Support function declarations in the definitions of other functions.

As part of this, function declarations are no longer scoped (this is permitted
by the C standard, as it turns out.)  So code like:

   void foo() { void bar(); }
   void bat() { bar(); }

Compiles correctly; the declaration of bar() in foo() is still available in the
definition of bar().

Fixes issue #129.

parse.yy

@@ -494,6 +494,7 @@ declaration_statement
             $$ = NULL;
         }
         else {
+            $1->DeclareFunctions();
             std::vector<VariableDeclaration> vars = $1->GetVariableDeclarations();
             $$ = new DeclStmt(vars, @1);
         }