chore(tvix/store): make importable

This allows other crates to import tvix_store.

Rename the bin crate to tvix-store-bin, to avoid having multiple crates
with the same name (https://github.com/rust-lang/cargo/issues/6313)

Change-Id: I857768d6115640dbf102e79ed03e8474090df2fe
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7728
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/lib.rs

@@ -0,0 +1,10 @@
+pub mod nixbase32;
+pub mod nixpath;
+pub mod proto;
+
+pub mod dummy_blob_service;
+pub mod dummy_directory_service;
+pub mod dummy_path_info_service;
+
+#[cfg(test)]
+mod tests;