Fix build on 0.11pre: Change ~"foo" to "foo".to_owned()

[~"string" and &"string" literals are gone](https://github.com/mozilla/rust/pull/13877), and were
replaced by "string".to_owned() and just "string", respectively. This commit was made by a simple
`sed` to convert the existing instances of ~"string" on the codebase.

For reference, the command used was this:

    sed -e 's/~\("[^"]*"\)/\1.to_owned()/g' -i *.rs

That wouldn't work if for strings that contain the `\"` escape in them, but this codebase had none.

Similar work is likely to be necessary after [~T is substituted by
Box<T>](https://github.com/mozilla/rust/issues/13885).