Create Rust workspace at the project root, and a Makefile.

[stephenctw]

Quote from: #78 (review)

I think it might make sense to make the root of the repo a Rust workspace. This should make everything "compatible" version-wise, specially regarding alloy. When you specify the members of a workspace, it's possible to put a path (like cartesi-rollups/node/reader). If we do this, only in the root should there be a Cargo.lock.

I think it may also make sense to create a Makefile at the root. One target should be to create the bindings (recursively calling the bindings makefile), and another target (depends on bindings) should build the Rust workspace. Other targets for CI like rustfmt, cargo test, cargo clippy (clippy should only be warnings for now). Other targets for building and testing smart contracts (which could be "recursive", in the sense that they call an inner makefile). I don't think the inner compute tests should have a target on this root makefile.