Fix metadata stats.

This commit:
- Counts some things that weren't being counted previously, and adds
  an assertion that ensure everything is counted.
- Reorders things so the `eprintln`s order matches the code order.
- Adds percentages, and makes clear that the zero bytes count is orthogonal to
  the other measurements.

Example of the new output:
```
55463779 metadata bytes, of which 18054531 bytes (32.6%) are zero
             preamble:       30 bytes ( 0.0%)
                  dep:        0 bytes ( 0.0%)
          lib feature:    17458 bytes ( 0.0%)
            lang item:      337 bytes ( 0.0%)
      diagnostic item:     1788 bytes ( 0.0%)
           native lib:        0 bytes ( 0.0%)
      foreign modules:     5113 bytes ( 0.0%)
       def-path table:   720180 bytes ( 1.3%)
               traits:      359 bytes ( 0.0%)
                impls:    64624 bytes ( 0.1%)
     incoherent_impls:      130 bytes ( 0.0%)
                  mir: 16137354 bytes (29.1%)
                 item: 23773099 bytes (42.9%)
interpret_alloc_index:      599 bytes ( 0.0%)
      proc-macro-data:        0 bytes ( 0.0%)
               tables: 10081135 bytes (18.2%)
 debugger visualizers:        0 bytes ( 0.0%)
     exported symbols:     5666 bytes ( 0.0%)
              hygiene:  1539390 bytes ( 2.8%)
      def-path hashes:  2752564 bytes ( 5.0%)
           source_map:   363540 bytes ( 0.7%)
                final:      413 bytes ( 0.0%)
```

0 files changed, 0 insertions, 0 deletions