Can't compile `rust-coroutine` with Rust 1.8.0

[circlespainter]

With Rust 1.8.0 installed by Linuxbrew on Linux Mint 17.4 Rosa x64 (based on Trusty), when running cargo build --release:

/home/fabio/.cargo/registry/src/github.com-88ac128001ac3a9a/fs2-0.2.2/src/unix.rs:63:67: 63:79 error: mismatched types:
 expected `i64`,
    found `u64` [E0308]
/home/fabio/.cargo/registry/src/github.com-88ac128001ac3a9a/fs2-0.2.2/src/unix.rs:63     let ret = unsafe { libc::posix_fallocate(file.as_raw_fd(), 0, len as off_t) };
                                                                                                                                                       ^~~~~~~~~~~~

Sorry, not knowledgeable enough in Rust to open a PR but wanted to give it a try nevertheless.