Make prelude create an empty _macro_ standalone

[gilch]

defmacros after prelude don't create the _macro_ namespace, because one was already detected at compile time. However, if Hissp isn't installed, the resulting Python should stand alone. prelude silently ignores the import error and doens't bother creating _macro_ at runtime in that case, but that doesn't correct the defmacros which still expand as if _macro_ exists. I've adjusted prelude to create an empty _macro_ namespace regardless, and then only populate it if it can import it from Hissp.