`Send` and `Sync`

What is the status about `Send` and `Sync`?
There is no explicit unimpl note, but due to the raw-pointers, it is also not auto-impled on `Proj`.
Is it safe to move the `Proj` instance between threads and have shared calls to `.project`?

If so, what about providing the impl:
```rust
unsafe impl Sync for Proj {}
unsafe impl Send for Proj {}
```

If not, what about clarifying it in the docs / README.

Background: I would like to load and parse the proj-definition once and call the projection from different places in the application.

Start with the README and the `Proj` API described in the issue, then trace how its raw pointers are managed and whether concurrent `.project` calls are supported. Done means reaching a clear safety decision and either providing the requested `Send`/`Sync` behavior with tests or documenting the restriction.

Written by the indexing model from the issue text.