Mucell often breaks Hyper

[pyfisch]

Mucell breaks Hyper really often. Like for example just now: https://travis-ci.org/hyperium/hyper/builds/47357537 I do not really know what mucell makes it superior to the standard library types, but for me it looks like it only provides a minor speed increasement. So do we really need it or could we switch to another builtin type?

[reem]

@pyfisch it's fixed now, but I do see your concern.

[seanmonstar]

I guess a benchmark (of hyper) using both mucell and refcell would be useful to know if it's worth using something outside of std.