Overhaul `-Zmeta-stats` output.

It's now much more like the `-Zhir-stats` output.
- Each line is preceded with `meta-stats`, which makes the provenance
  clearer and allows filtering of the output.
- Sections are now sorted in reverse order of size.
- Column headings avoid the need to repeat the word "bytes" on every line.
- Long numbers now have `_` separators for easier reading.
- Consistent use of '-' within section labels, rather than a mix of '-',
  '_', and ' '.

The code itself is shorter and easier to read thanks to:
- the `stat` macro, which encapsulates each section's encoding, avoids
  some boilerplate, and removes the need for some low-value comments;
- the `stats` vector, which replaces dozens of local variables.

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