Port late master additions to weak_mem

Since the paper has been published, I made a few small additions to master as questions came up in discussions or as the standard library changed:

https://gitlab.mpi-sws.org/FP/LambdaRust-coq/commit/505d86a5fbb85abd4e5fada1853136ac690a60bf: &mut T <-> &mut Cell<T>
https://gitlab.mpi-sws.org/FP/LambdaRust-coq/commit/ff244cf1b7deabfd9bc1e4eae0bc48db6aedbe77: Box<T> <-> Box<Cell<T>>
https://gitlab.mpi-sws.org/FP/LambdaRust-coq/commit/7da18513b7d41b50cc58cd08136115d59d42828e: impl<T> Send for JoinHandle<T>
e6a4bef4: Faking a &&mut.

@haidang @jjourdan which of these still need porting to weak_mem?

Edited Aug 20, 2018 by Hai Dang

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