Added stubs for SplDoublyLinkedList, SplQueue and SplPriorityQueue

[dktapps]

I made these for a project I'm working on, perhaps they'll be useful to someone else :)

NB: SplPriorityQueue is very nasty because the return types depend on the extract flags, which aren't available in the type system. It might be necessary to implement a dynamic return type extension for those.

[ondrejmirtes]

Merged, thank you! 6a38951

It might be necessary to implement a dynamic return type extension for those.

Yes, please do that :)

[dktapps]

I had a think about how to do this and the problem I keep running into is that the information needed to decide what the extract flags of PriorityQueue are is not present in the type system. If an SplPriorityQueue is passed to a function parameter, the callee doesn't know from the type system what the extract flags are.
@ondrejmirtes do you have any idea how such a problem can be solved? I'm dubious whether it's worth the time tbh; I may just create wrapper classes which seal this nastiness away (or use Ds\PriorityQueue instead).

[ondrejmirtes]

Can you show a piece of code that demonstrates the problem you're facing?