Add attribute to specify symbol name, as an alternative to no_mangle

[ids1024]

I suggested this in rust-lang/rust#42202, to address an issue mentioned there (namely, symbol names that are not valid identifiers in Rust), but there are other uses for such a feature. Certainly it would be helpful if symbols need to be exposed to another language with some sort of mangling but also called from Rust with less ugly names. I also seem to vaguely recall wanting such a feature for some other reason, but I can't recall exactly why.

There are several possible syntaxes:

• #[symbol="some_symbol_name"]
• #[no_mangle="some_symbol_name"]
• Something else

I can't think of any major disadvantages, other than the ones that are already present by allowing no_mangle. And of course the general disadvantage of adding a relatively obscure feature.

[ids1024]

Oh wait, I think export_name does this.

[eddyb]

Pretty sure it's link_name?

[ids1024]

link_name - the name of the symbol that this function or static should be imported as.
export_name - on statics and functions, this determines the name of the exported symbol.
https://doc.rust-lang.org/stable/reference/attributes.html

It looks to me like these do similar things, the fist when importing a function, the second when exporting a function. Is that wrong? The documentation is somewhat sparse here (these are, after all, fairly obscure and rarely needed features).