Add some timing info to rustdoc

There are various improvements, but the main one is to time each pass
that rustdoc performs (`rustdoc::passes`).

Before, these were the top five timings for `cargo doc` on the cargo
repository:

```
+---------------------------------+-----------+-----------------+----------+------------+
| Item                            | Self time | % of total time | Time     | Item count |
+---------------------------------+-----------+-----------------+----------+------------+
| <unknown>                       | 854.70ms  | 20.888          | 2.47s    | 744823     |
+---------------------------------+-----------+-----------------+----------+------------+
| expand_crate                    | 795.29ms  | 19.436          | 848.00ms | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
| metadata_decode_entry           | 256.73ms  | 6.274           | 279.49ms | 518344     |
+---------------------------------+-----------+-----------------+----------+------------+
| resolve_crate                   | 240.56ms  | 5.879           | 242.86ms | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
| hir_lowering                    | 146.79ms  | 3.587           | 146.79ms | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
```

Now the timings are:

```
+---------------------------------+-----------+-----------------+----------+------------+
| Item                            | Self time | % of total time | Time     | Item count |
+---------------------------------+-----------+-----------------+----------+------------+
| <unknown>                       | 1.40s     | 22.662          | 3.73s    | 771430     |
+---------------------------------+-----------+-----------------+----------+------------+
| collect-trait-impls             | 1.34s     | 21.672          | 2.87s    | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
| expand_crate                    | 1.21s     | 19.577          | 1.28s    | 1          |
+---------------------------------+-----------+-----------------+----------+------------+
| build extern trait impl         | 704.66ms  | 11.427          | 1.07s    | 21893      |
+---------------------------------+-----------+-----------------+----------+------------+
| metadata_decode_entry           | 354.84ms  | 5.754           | 391.81ms | 544919     |
+---------------------------------+-----------+-----------------+----------+------------+
```

The goal is to help me debug regressions like rust-lang#74518 (comment) (currently I have _no_ idea what could have gone wrong).

r? @eddyb or @Mark-Simulacrum