Stabilize `-Ztimings` as `--timings`

Stabilize `-Ztimings` as `--timings`

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.