Function bridges

pub fn bridges(
    rust_source_files: impl IntoIterator<Item = impl AsRef<Path>>,
) -> Build

cxx_build::bridge but for when more than one file contains a #[cxx::bridge] module.

let source_files = vec!["src/main.rs", "src/path/to/other.rs"];
cxx_build::bridges(source_files)
    .file("src/demo.cc")
    .std("c++11")
    .compile("cxxbridge-demo");