RISC-V PVM: simple storage backend

What

A simple storage system for the RISC-V PVM. Allows creating/loading a disk-backed repo, to which arbitrary data can be committed.

Why

We need a simple storage backend for the (dummy, for now) RISC-V PVM.

How

At commit time, the data is chunked and any previously unseen chunks are written to disk along with a commit file listing its chunks. When checking out, data is recreated from the commit file and the chunks.

Manually testing the MR

cargo test -- test_repo