Auto merge of #6802 - nnethercote:report-kind, r=jdm

Add a `kind` field to memory reports.

This is used for two memory reporting improvements.

- It's used to distinguish "explicit" memory reports from others. This
  mirrors the same categorization that is used in Firefox, and gives a single
  tree that's the best place to look. It replaces the "pages" tree which
  was always intended to be a temporary stand-in for "explicit".

- It's used to computed "heap-unclassified" values for both the jemalloc
  and system heaps, both of which are placed into the "explicit" tree.

Example output:
```
|  114.99 MiB -- explicit
|      52.34 MiB -- jemalloc-heap-unclassified
|      46.14 MiB -- system-heap-unclassified
|      14.95 MiB -- url(file:///home/njn/moz/servo2/../servo-static-suite/wikipe
dia/Guardians%20of%20the%20Galaxy%20(film)%20-%20Wikipedia,%20the%20free%20encyc
lopedia.html)
|          7.32 MiB -- js
|             3.07 MiB -- malloc-heap
|             3.00 MiB -- gc-heap
|                2.49 MiB -- used
|                0.34 MiB -- decommitted
|                0.09 MiB -- unused
|                0.09 MiB -- admin
|             1.25 MiB -- non-heap
|          1.36 MiB -- layout-worker-3-local-context
|          1.34 MiB -- layout-worker-0-local-context
|          1.24 MiB -- layout-worker-1-local-context
|          1.24 MiB -- layout-worker-4-local-context
|          1.16 MiB -- layout-worker-2-local-context
|          0.89 MiB -- layout-worker-5-local-context
|          0.38 MiB -- layout-task
|             0.31 MiB -- display-list
|             0.07 MiB -- local-context
|       1.56 MiB -- compositor-task
|          0.78 MiB -- surface-map
|          0.78 MiB -- layer-tree
```
The heap-unclassified values dominate the "explicit" tree because reporter
coverage is still quite poor.

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