Use pwritev for positional scatter-gather disk IO

[vimpunk]

Only (non-positional) scatter-gather IO is supported in the std lib via the Write::write_vectored trait interface, but this is not good enough for our use case as we'll need to perform 2 syscalls for every write: seek and writev.

• Rust stdlib feature request: Support positional vectored IO for unix files rust-lang/rust#71055
• Possible workaround until then: https://github.com/vasi/positioned-io/tree/positioned-io-preview [EDIT]: this doesn't work as it only supports positional IO, not scatter-gather IO.
• or: https://docs.rs/nix/0.17.0/nix/sys/uio/fn.pwritev.html