This puts the larger sub-trees first. E.g. this:
```
| 1.04 MiB -- url(http://en.wikipedia.org/wiki/Main_Page)
| 0.26 MiB -- display-list
| 0.78 MiB -- paint-task # new output line
| 0.78 MiB -- buffer-map # new output line
```
becomes this:
```
| 1.04 MiB -- url(http://en.wikipedia.org/wiki/Main_Page)
| 0.78 MiB -- paint-task # new output line
| 0.78 MiB -- buffer-map # new output line
| 0.26 MiB -- display-list
```
This matches how Firefox's about:memory works.
Now that this is done for all sub-trees, the ad hoc sorting done for
Linux segments is no longer necessary, and has been removed.
A rebuild after touching components/profile/mem.rs now takes 48 seconds (and
only rebuilds `profile` and `servo`) which is much lower than it used to be.
In comparison, a rebuild after touching components/profile_traits/mem.rs takes
294 seconds and rebuilds many more crates.
This change also removes some unnecessary crate dependencies in `net` and
`net_traits`.
- Most of util::memory has been moved into profile::mem, though the
`SizeOf` trait and related things remain in util::memory. The
`SystemMemoryReporter` code is now in a submodule
profile::mem::system_reporter.
- util::time has been moved entirely into profile::time.
