Performance ideas from the top of my head

[benjamingr]

Here are some performance ideas based on a cursory glimpse of the code based on perf work from bluebird:

• Store tasks in a deque instead of an array or a list
• Trampoline async tasks and allocate fewer promises (don't allocate new promises unless needed basically)
• Don't use a linked list for reusify, use a deque

[mcollina]

Last time I checked the current reusify implementation was the one that yielded the best results due to the reduced allocations/gc pressure. Usually linked lists or arrays store two objects, one for the node of the list and one for the properties you want to contain. Reusify collapses those into one single object, cutting in half the allocations.

I've not optimized the promise version as much so there is plenty of optimizations potential there, PRs are welcomed.