Remove box pattern

[faern]

Hi!

Here is a way to destructure the boxes without the unstable box pattern. Makes all the tests pass on stable Rust.

Not fully sure I didn't miss any strange case since this is unsafe and all. But maybe you can spot any potential problems?

[coveralls]

Coverage decreased (-0.02%) to 98.315% when pulling a2410ed on faern:remove-box-pattern into a6533f5 on jeromefroe:master.

[faern]

Here is a text about why this very special moving dereferencing works on Box: https://manishearth.github.io/blog/2017/01/10/rust-tidbits-box-is-special/