Rename the binary to be garden

In `./target/debug/` we can see the binary name:
`digital-garden-steps`. This is the name of our package
and we want the actual binary that users install to be
named `garden` instead.

To do this we can change the auto-discovered `src/main.rs`
binary and change the name using a `bin` config in the
`Cargo.toml` file.

```toml
[[bin]]
name = "garden"
path = "src/main.rs"
```

Then we see `./target/debug/garden` get created when we
`cargo run` or `cargo build`.

Cargo.toml

@@ -6,4 +6,8 @@ edition = "2018"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[[bin]]
+name = "garden"
+path = "src/main.rs"
+
 [dependencies]