[Cython] A small change in compiler directive implementation

[Dag Sverre Seljebotn]

Just a note that I've changed some internals with compiler directives.

Before each node would get an "options" attribute in a transform that 
ran pretty early. This was getting increasingly cumbersome to deal with 
(as transforms needed to make sure options was copied over to new nodes 
etc.). So now I've created a CompilerDirectivesNode that sit in the tree 
  instead. Each recursive process then stores the directives these sets 
in different places:

- env.directives
- code.globalstate.directives
- For CythonTransform descendants, self.current_directives is 
automatically updated as long as you leave the default 
CompilerDirectivesNode visit alone.

If anyone protests I'll take it upon me to back it out.

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Dag Sverre