Make modules system consistent

[kaichaosun]

There are 2 styles when introduce modules in Rust, following are the structure with a chat module:

Old style (with mod.rs),

src
 |- chat
        |- mod.rs
        |- other.rs
 |- lib.rs
 |- xyz.rs

New style:

src
 |- chat
        |- other.rs
 |- lib.rs
 |- xyz.rs
 |- chat.rs

Signal your preference so we can decide which styles to use. After we have all the dev signals, change the codebase to use the agreed style.