Optimize building speed (#442)

Make easytier-cli and easytier-core link to the easytier library to
avoid duplicate linking of mods.

easytier/src/lib.rs

@@ -5,13 +5,16 @@ mod connector;
 mod gateway;
 mod instance;
 mod peer_center;
-mod peers;
 mod vpn_portal;
 
 pub mod common;
 pub mod launcher;
+pub mod peers;
 pub mod proto;
 pub mod tunnel;
 pub mod utils;
 
+#[cfg(test)]
+mod tests;
+
 pub const VERSION: &str = common::constants::EASYTIER_VERSION;