Support for multiple libraries

[temeddix]

Hi. First of all, I would like to thank you for this amazing library. It's an amazing job.

I was wondering if this library could support generation of multiple libraries(.dll/.so) so that it can be properly used in a huge project. Right now, it seems possible to split API modules into api_1.rs, api_2.rs, but eventually it just gets compiled into a single library file. Wouldn't it be great to support multiple libraries, which is the same as multiple crates inside /native?

[fzyzcjy]

You are welcome! The suggestion looks quite reasonable, so feel free to PR :)

Also cc @dbsxdbsx who implemented the nice multi-file feature