Update nix requirement from 0.16 to 0.18

[dependabot-preview]

Updates the requirements on nix to permit the latest version.

Changelog

Sourced from nix's changelog.

[0.18.0] - 26 July 2020

Added

• Added fchown(2) wrapper. (#1257)
• Added support on linux systems for MAP_HUGE_SIZE family of flags. (#1211)
• Added support for F_OFD_* fcntl commands on Linux and Android. (#1195)
• Added env::clearenv(): calls libc::clearenv on platforms where it's available, and clears the environment of all variables via std::env::vars and std::env::remove_var on others. (#1185)
• FsType inner value made public. (#1187)
• Added unistd::setfsuid and unistd::setfsgid to set the user or group identity for filesystem checks per-thread. (#1163)
• Derived Ord, PartialOrd for unistd::Pid (#1189)
• Added select::FdSet::fds method to iterate over file descriptors in a set. (#1207)
• Added support for UDP generic segmentation offload (GSO) and generic receive offload (GRO) (#1209)
• Added support for sendmmsg and recvmmsg calls (#1208)
• Added support for SCM_CREDS messages (UnixCredentials) on FreeBSD/DragonFly (#1216)
• Added BindToDevice socket option (sockopt) on Linux (#1233)
• Added EventFilter bitflags for EV_DISPATCH and EV_RECEIPT on OpenBSD. (#1252)
• Added support for Ipv4PacketInfo and Ipv6PacketInfo to ControlMessage. (#1222)
• CpuSet and UnixCredentials now implement Default. (#1244)
• Added unistd::ttyname (#1259)
• Added support for Ipv4PacketInfo and Ipv6PacketInfo to ControlMessage for iOS and Android. (#1265)
• Added support for TimerFd. (#1261)

Changed

• Changed fallocate return type from c_int to () (#1201)
• Enabled sys::ptrace::setregs and sys::ptrace::getregs on x86_64-unknown-linux-musl target (#1198)
• On Linux, ptrace::write is now an unsafe function. Caveat programmer. (#1245)
• execv, execve, execvp and execveat in ::nix::unistd and reboot in ::nix::sys::reboot now return Result<Infallible> instead of Result<Void> (#1239)
• sys::socket::sockaddr_storage_to_addr is no longer unsafe. So is

Commits

• 96054b6 Merge #1277
• 59615ed Release version 0.18.0
• 2a668fa Merge #1270
• cbcc42d Support nullable timeout in ppoll
• 557a8fe Merge #1276
• bdc416c Raise libc dependecy to 0.2.73
• 57b6bd7 Merge #1224
• b1ba074 Merge #1261
• f3bd6ed Adding an implementation and some basic tests for timerfd.
• 4b6b14a Merge #1265
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #75.