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/lib.rs

@@ -1,3 +0,0 @@
-pub mod protos {
-    include!(concat!(env!("OUT_DIR"), "/tvix.store.v1.rs"));
-}