Add unsound const-cstr

[oherrala]

const-cstr is crate by @abonander who has archived bunch of his Rust crates in GitHub.

See communication attempts on other crates:

• buf_redux might be unmaintained #1602
• multipart Status #1438

[Nugine]

Alternatives:

• const_str::cstr!: decl macro + const evaluation
• cstr::cstr!: proc macro

[pinkforest]

btw const_cstr doesn't check for any interior nul bytes

Both the constructor and the canonical macro allows this:

const_cstr! {
    FOO = "\u{0}💖\u{0}";
}

let foo = FOO.as_cstr();

This violates the safety contract of ffi::CStr::from_bytes_with_nul_unchecked that is used in as_cstr()

ConstCStr::as_cstr assert only checks the last byte is nul that is relied for safety:
https://github.com/abonander/const-cstr/blob/master/src/lib.rs#L122

Also because unicode(tm) vs bytes - as_str goes:

"\x00\xf0\x9f\x92\x96\x00" vs ConstCStr { val: "\0💖\0" }

[pinkforest]

Ok I've filled the advisory with some details - please check if anything missed. Thanks

[oherrala]

@pinkforest LGTM 👍