add empty workspace

[Neutron3529]

This ensures the output dir match the one defined in Makevars.

.
├── Cargo.toml
├── LICENSE
├── README.md
├── rmin
│  ├── build.rs
│  ├── Cargo.toml
│  └── src
│     ├── base.no-std.rs
│     ├── base.no-std.unwind.rs
│     ├── base.rs
│     ├── base.s.r-type.lib-r.rs
│     ├── base.s.r-type.rs
│     ├── base.s.rs
│     └── lib.rs
├── rmin-eh
│  ├── Cargo.toml
│  └── src
│     ├── dwarf
│     ├── emcc.rs
│     ├── gcc.rs
│     └── lib.rs
├── rmin-examples
│  ├── Cargo.toml
│  ├── R-package-demo
│  │  └── rext
│  └── src
│     ├── compare-rextendr.rs.txt
│     └── lib.rs
└── rmin-macros
   ├── Cargo.toml
   └── src
      └── lib.rs

Suppose I'm writting something under a workspace (see the structure above).

The Cargo.toml in the root contains a [workspace] unit, and thus all the inner Cargo.toml file is affected.

As for the rmin-examples/R-package-demo/rext/src/rust crate, It is affected and thus may failed to compile:

$ cargo build
error: current package believes it's in a workspace when it's not:
current:   /me/rmin/rmin-examples/R-package-demo/rext/src/rust/Cargo.toml
workspace: /me/rmin/Cargo.toml

this may be fixable by adding `rmin-examples/R-package-demo/rext/src/rust` to the `workspace.members` array of the manifest located at: /me/rmin/Cargo.toml
Alternatively, to keep it out of the workspace, add the package to the `workspace.exclude` array, or add an empty `[workspace]` table to the package's manifest.

As it suggests, adding an empty [workspace] solve this problem, this is the reason why I open this PR.

[CGMossa]

Thank you for your pull request! Unfortunately, I don't see a clear argument, as to why ever rust-crate in an R-package must be a workspace. For that reason, I'm closing this PR.