extract varint/zigzag stuff to separate module

They can be used for more than recording. In particular, I plan to use these
from the db module for the representation of signals/events.

src/main.rs

@@ -69,6 +69,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::thread;
 
 mod clock;
+mod coding;
 mod db;
 mod dir;
 mod error;