Remove Fibers dependency

[perbergland]

For Meteor 2.9 or 3.0, this package needs to be updated to remove its direct import of Fiber If needed, replace the Fiber.current/ new Fiber() calls with something that works in a fibers-free world

[StorytellerCZ]

Given https://github.com/meteor/meteor/pull/12442 this package will probably not be needed for Meteor 3.0.

[copleykj]

This is in progress, mostly because fast-render depends on this and I don't feel like trying to reproduce the functionality with express alone.