Parallelize get, install & update #586
Description
Parallelize get, install & update.
Parallelization is more complex for these commands than for other commands, because they update existing terminal lines to update process bars. Currently, mas just updates the current line, but parallel downloads will require different lines being updated depending on which app's progress is being updated.
When an error occurs for a single parallel operation, other operations that can succeed should be let finish.
Probably not worth parallelizing uninstall, as that should be quick & not run much.
Swift Concurrency: async let, TaskGroup & withTaskGroup.