explore: consolidate padding config, handle ByteStream, tweak naming+comments

[rgwood]

Some minor changes to explore, continuing on my mission to simplify the command in preparation for a larger UX overhaul:

1. Consolidate padding configuration. I don't think we need separate config points for the (optional) index column and regular data columns in the normal pager, they can share padding configuration. Likewise, in the binary viewer all 3 columns (index, data, ASCII) had their left+right padding configured independently.
2. Update explore so we use the binary viewer for the new ByteStream type. cat foo.txt | into binary | explore was not using the binary viewer after the ByteStream changes.
3. Tweak the naming of a few helper functions, add a comment

I've put the changes in separate commits to make them easier to review.

[sholderbach]

Awesome, thanks for simplifying this. Having a tighter configuration with a few obvious knobs should make this more usable.

Haven't run it yet (so can't say much about the ByteStream stuff, cc @devyn , @IanManske )

[devyn]

Seems to work, but I can't use page up and page down. I'm using:

open ~/.cargo/bin/nu | into binary | explore

[rgwood]

@devyn I think that's a preexisting issue, I can't use page up/down in the binary viewer on an older version of Nushell either. I think there's a lot of stuff that isn't quite finished in the binary viewer :(