Enable new parser by default (!). Fixes #557.

This entirely replaces fish's execution model. Bugs are likely. See commit 290aae80 for what this is about.

parser.cpp

@@ -3127,7 +3127,7 @@ bool parser_use_ast(void)
     env_var_t var = env_get_string(L"fish_new_parser");
     if (var.missing_or_empty())
     {
-        return 0;
+        return 1;
     }
     else
     {