atomic-dbg

This crate provides [dbg], [eprint], and [eprintln], macros which work just like their counterparts [std], but which:

• Write atomically, up to the greatest length supported on the platform.
• Don't use locks (in userspace).
• Preserve libc's errno and Windows' last-error code value.

This means they can be used just about anywhere within a program, including inside allocator implementations, inside synchronization primitives, startup code, around FFI calls, inside signal handlers, and in the child process of a fork before an exec.

And, when multiple threads are printing, as long as their within the length supported on the platform, the output is readable instead of potentially interleaved with other output.

For example, this code: ```rust use atomic_dbg::dbg;

fn main() { dbg!(2, 3, 4); } ```

Has this strace output: write(2, "[examples/dbg.rs:4] 2 = 2\n[examples/dbg.rs:4] 3 = 3\n[examples/dbg.rs:4] 4 = 4\n", 78[examples/dbg.rs:4] 2 = 2 which is a single atomic write call.

For comparison, with std::dbg it looks like this: write(2, "[", 1[) = 1 write(2, "examples/dbg.rs", 15examples/dbg.rs) = 15 write(2, ":", 1:) = 1 write(2, "4", 14) = 1 write(2, "] ", 2] ) = 2 write(2, "2", 12) = 1 write(2, " = ", 3 = ) = 3 write(2, "2", 12) = 1 write(2, "\n", 1 ) = 1 write(2, "[", 1[) = 1 write(2, "examples/dbg.rs", 15examples/dbg.rs) = 15 write(2, ":", 1:) = 1 write(2, "4", 14) = 1 write(2, "] ", 2] ) = 2 write(2, "3", 13) = 1 write(2, " = ", 3 = ) = 3 write(2, "3", 13) = 1 write(2, "\n", 1 ) = 1 write(2, "[", 1[) = 1 write(2, "examples/dbg.rs", 15examples/dbg.rs) = 15 write(2, ":", 1:) = 1 write(2, "4", 14) = 1 write(2, "] ", 2] ) = 2 write(2, "4", 14) = 1 write(2, " = ", 3 = ) = 3 write(2, "4", 14) = 1 write(2, "\n", 1 ) = 1