robin-hood HashMap "tail-call" should be rewritten as a loop

[pnkfelix]

This code:

https://github.com/mozilla/rust/blob/2eebeb81372e320510a1c1e2eef96eb5146a1e1f/src/libcollections/hashmap.rs#L1099

is believed to be causing this stack overflow:

https://gist.github.com/Meyermagic/9523484

Some cursory analysis indicates that this code shouldn't need to be using (an attempt at) tail recursion; a version that uses a loop shouldn't be a difficult refactoring. See discussion here:

https://botbot.me/mozilla/rust/msg/12053161/

[Meyermagic]

The following reduced example causes the behavior in question: https://gist.github.com/Meyermagic/9524658