DSL avoid checking embedded Python objects

[cjdoris]

We embed PyInternedStrings and PyLazyObjects into the AST, but accessing them still does a NULL check and IncRef each time.

Instead, we could keep a list of all the lazy objects used, and a bool indicating whether or not they have all been checked, and check this once at the start of the DSL block. So the first time, all these objects are initialised, and thereafter we can just extract the pointer value directly.