Fixes for rustc master

[bkoropoff]

I'm submitting this PR for feedback, but it's not yet ready to merge. The following PRs on upstream projects need to land first:

• Relax lifetime bounds on Reader/Writer impls for trait boxes rust#16843
• Fix build break due to generalized parameter bounds servo/rust-url#22
• Fix build break due to generalized type parameter bounds docopt/docopt.rs#15
• Fix build break due to generalized object bounds lifthrasiir/rust-encoding#39

Once these land I will update Cargo.lock and add it to the PR

[alexcrichton]

In shell.rs, could you use 'static in the relevant locations instead of an arbitrary 'a? That would allow us to land this more quickly (once rust-url/rust-encoding have been merged).

[bkoropoff]

@alexcrichton test_shell.rs actually calls Shell::create() with a BufWriter, which is what necessitated this. Changing the bound to 'static would work if the unit test were commented out.

[alexcrichton]

Hm, could that change to a MemWriter and/or a ChanReader or ChanWriter? I'd like to merge this today if possible instead of waiting for the next nightly.

[bkoropoff]

Calling unwrap() on a Shell simply fails, so I can't get the MemWriter back out. I can call get_ref(), but that gives me a ref to a trait box, and there is no method on Writer I can use to peek at the data in the MemWriter. I haven't had coffee yet, though, so maybe I'm missing something.

[bkoropoff]

I almost got ChanWriter to work, but I'm getting a hang in one of the tests unless I adjust the vector to have a precise size, which seems like it should be unnecessary, and would probably break Windows if I did it. I won't have time to look at this again until later tonight, so I left in a branch if someone else wants to pick it up:

https://github.com/bkoropoff/cargo/tree/whatsit