WISH: Deploy futurize transpilation on package load

Add a function `futurize_apply_to_package()` that can be used in a package's `.onLoad()` to apply all `futurize()` transpilations on package load, e.g.

```r
.onLoad <- function(libname, pkgname) {
futurize::futurize_apply_to_package()
}
```

Special care has to be take for arguments such as `when` and `eval`.

# Benefits

* It removes the tiny overhead of the `futurize()` transpiler that otherwise applies at each call.
* It validates that all `futurize()` transpilations are valid up front.
* ...?

Trace the existing `futurize()` implementation and the package `.onLoad()` entry point first, focusing on how arguments such as `when` and `eval` are handled. Done means a `futurize_apply_to_package()` function can apply all package transpilations during load and validate them up front.

Written by the indexing model from the issue text.