Allow access to bindings symbols names

[TheElectronWill]

What I'm trying to do:

1. write a Rust library with a C-compatible interface (extern functions, etc)
2. use cbindgen to generate a C header, to use the library from C
3. compile a Rust program that depends on the library and links with -C link-args=-Wl,--export-dynamic-symbol=<regex of symbols to export> (or another ldd option), in order to load plugins (written in C) that use the library symbols

For step 3, I need the list of symbols in the C header. I can parse the header or fill this information by hand, but cbindgen already has the list! What is missing is a way to access it.

I can see two ways of solving this problem (both may be useful):

1. Modify the Bindings impl: add a public API to read the items of the bindings (with methods to get the exported name of an item of any type), for instance:

   let bindings = cbindgen::Builder::new().with_crate(crate_dir).with_language(C).generate().unwrap(); // usual generation
   let symbols_names: Vec<String> = bindings.items.iter().map(|i| i.export_name()).collect(); // would be awesome if possible

2. Add a way to export the symbols in the linker format, which looks like

   {
   symbol_1;
   symbol_2;
   };

   I will then be able to use --export-dynamic-symbol-list=<exported file> in order to export the symbols properly.

If someone guides me a little bit, I could try to implement one of the aforementioned features (or both, who knows?) :slightly_smiling_face:.