Optimize building speed (#442)

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

easytier/build.rs

@@ -147,7 +147,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         .type_attribute("peer_rpc.ForeignNetworkRouteInfoKey", "#[derive(Hash, Eq)]")
         .type_attribute("common.RpcDescriptor", "#[derive(Hash, Eq)]")
         .service_generator(Box::new(rpc_build::ServiceGenerator::new()))
-        .btree_map(&["."])
+        .btree_map(["."])
         .compile_protos(&proto_files, &["src/proto/"])
         .unwrap();