Content addressable storage

targets 1.8.0

• Wrap tar_watch() UI module in bslib::page() (#1302, @kwbyron-lilly).
• Remove callr_function in tar_make_as_job() argument list.
• Ensure storage = "worker" is respected when the process of storing an object generates an error (#1304, @multimeric).
• Default to the _targets.R pattern in tar_branches() (#1306, @multimeric, @mattwarkentin).
• Remove superfluous functions and globals from metadata with tar_prune() (#1312, @benzipperer).
• Change the default workspace_on_error option to TRUE (#1310, @hadley).
• Enhance and organize the error = "stop" error message.
• Avoid saving a file in _targets/objects for error = "null". Instead, switch to a special "null" storage format class if error is "null" the target throws an error. This should allow users to more freely create new formats with tar_format() without worrying about how to handle NULL objects created by error = "null".
• Implement format = "auto" (#1311, @hadley).
• Replace pingr dependency with base::socketConnection() for local URL utilities (#1317, #1318, @Adafede).
• Implement tar_repository_cas(), tar_repository_cas_local(), and tar_repository_cas_local_gc() for content-addressable storage (#1232, #1314, @noamross).
• Add tar_format_get() to make implementing CAS systems easier.
• Implement error = "trim" in tar_target() and tar_option_set() (#1310, #1311, @hadley).
• Use the file system type to decide whether to trust time stamps (#1315, @hadley, @gaborcsardi).
• Deprecate format = "file_fast" in favor of the above (#1315).
• Deprecate trust_object_timestamps in favor of the more unified trust_timestamps in tar_option_set() (#1315).
• Print storage size of each target in verbose reporters (#1337, @psychelzh).
• Combine help files of tar_target() and tar_target_raw(). Same with tar_load() and tar_load_raw().
• Add a substitute argument to tar_format() to make it easier to write custom storage formats without metaprogramming.