Make static `ns_string!`s work inside closures

[madsmtm]

They don't currently for some reason!

Discovered in #254.

[madsmtm]

Similar error when using:

RUSTFLAGS="-Clink-dead-code" cargo build --example basic_usage

[madsmtm]

I've cfg-gated the functionality behind the feature flag unstable-static-nsstring for now: #350

[madsmtm]

objrs adds special export_names, so perhaps that's required?

[madsmtm]

A change in rustc dead code elimination causes the UTF16 to appear in the assembly output, even though it's known statically that the symbol is unused.

Note that this happens even if the #[link_section = "..."] attribute is removed.

Will probably need to bisect rustc to find the regression, it's somewhere between nightly-2023-05-25 and nightly-2023-08-27.

[madsmtm]

I bisected it to rust-lang/rust#115126, probably rust-lang/rust#115114, but also found that it only appears if you place the string in another static, not if you just use it as an expression.

So probably not worth doing more about.