Added the "localtest" directory for testing the API without touching
other modules. (TODO: It could be moved somewhere else, but should not
use src as an extern crate, or it will depend on the entire crate.)

Use CStr for incoming strings from C. CString is for C-style strings
**owned by Rust**.

Added comments at the beginning of api_c.rs and api_bridge.rs to
indicate some parts of the files are generated.