POSIX error numbers for Rust

This library defines a single type, the Error enum, which represents the symbolic constants for error numbers defined in the POSIX standard.

To depend on posix-errno from a Bazel workspace:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "rust_posix_errno",
    sha256 = "...",
    urls = ["https://github.com/jmillikin/rust-posix-errno/releases/download/v1.0.0/posix-errno-v1.0.0.tar.xz"],
)

To depend on posix-errno from a Cargo workspace:

[dependencies]
posix-errno = { version = "1.0.0" }