std.Progress: add byte unit flag and resource scope; support throughput calculation

[andrewrk]

Extracted from #20059.

The idea here is that you would be able to set a flag on a Node that indicates the units are byte. This would make it display, for example,

[6.5 MiB / 10 MiB] Downloading (192 KB/s)

rather than

[6815744/10485760] Downloading

for such a node.

It would also be nice if there was a resource scope. This way the root node could aggregate throughput based on scope and then display something like:

root name (down: 150 KB/s, up: 50 KB/s, disk: 100 MiB/s)

[VisenDev]

Perhaps allowing a custom unit name parameter could also accomplish similar goals, rather than mandating that the units are either bytes or unspecified.

Or if specific formatting of different unit types is required (such as the byte units being displayed as megabytes or kilobytes), an enum of several different possible unit types could be used.

[jcollie]

It would be nice if you could specify using 1024-based or 1000-based multipliers (MiB vs MB).