Worker globals

[dfalbel]

This PR introduces 2 new parameters to the dataloader function:

• worker_globals: objects that should be copied to the global environment of each worker.
• worker_packages: names of packages that should be loaded with library into each worker.

The code wouldn't work before, because the hello_fn wouldn't be available in the workers:

ds <- dataset(
    .length = function() {20},
    initialize = function() {},
    .getitem = function(id) {
      hello_fn()
    }
  )
  
dl <- dataloader(ds(), batch_size = 10, num_workers = 2)

You can now create the dataloader with:

dl <- dataloader(ds(), batch_size = 10, num_workers = 2, worker_globals = list(
    hello_fn = hello_fn
  ))

Fix for #444

cc @Athospd