build: make a top-level workspace

Creates a workspace with the following structure:

neptune-cash
├── Cargo.lock
├── Cargo.toml
├── neptune-cli
│   ├── Cargo.toml
│   └── src
│       └── main.rs
├── neptune-core
│   ├── Cargo.toml
│   └── src
│       ├── lib.rs
│       └── main.rs
├── neptune-dashboard
│   ├── Cargo.toml
│   └── src
│       └── main.rs
└── README.md

Author: dan-da

.gitignore

@@ -8,7 +8,7 @@ target/
 /makefile-target-opt-level3
 
 # git shouldn't see the proof files. Bc big.
-/test_data/*.proof
+/neptune-core/test_data/*.proof
 
 # Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
 # More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html