This avoids dropping them wherever Cargo happens to run from, and
instead places them under the target directory.
This has the advantage, and disadvantage, that `cargo clean` will remove
them.
The `-Ztimings` option has existed for years, and many people use it to
profile and optimize their builds. It's one of the common reasons people
use nightly cargo.
The machine-readable JSON output may warrant further careful inspection
before we commit to a stable format. However, for the human-readable
output we don't need to make any commitment about the exact output.
Add a `--timings` option, as the stable equivalent to `-Ztimings`.
Stabilize the `html` output format, and require `-Zunstable-options` for
the `json` output format.
Document the new option, and update the testsuite.
The text-based timing information emits many additional lines, creating
quite a bit of verbosity. Remove in favor of the HTML report, as
suggested at
https://github.com/rust-lang/cargo/pull/10245#issuecomment-1003741052 .
If we re-add text-based timing information in the future, it could come
in the form of a text-based report, or as a duration printed on the same
line as the crate it measures rather than a separate line.
