feat(tvix/store): add size() and digest() functions for Directory

Change-Id: I3a9e3c77da1d5d7bfb93da0cd7bce8d48dbc8cd4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7254
Reviewed-by: Adam Joseph <adam@westernsemico.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/store/src/main.rs

@@ -1,3 +1,5 @@
+mod proto;
+
 fn main() {
     println!("Hello, world!");
 }