This adds a new `Cargo.toml` feature named `deprecated` that opts
controls whether deprecation warnings show up. This is starting off as
non-default though that may change (see below).
Benefits
- Allows a staged rollout so a smaller subset of users see new
deprecations and can report their experience with them before everyone
sees them. For example, this reduces the number of people who have to
deal with #3822.
- This allows people to defer responding to each new batch of
deprecations and instead do it at once. This means we should
reconsider #3616.
The one risk is people who don't follow blog posts and guides having a
harder time upgrading to the next breaking release without the warnings
on by default. For these users, we reserve the right to make the
`deprecated` feature `default`. This is most likely to happen in a
minor release that is released in conjunction with the next major
release (e.g. when releasing 4.0.0, we release a 3.3.0 that enables
deprecations by default). By using a feature, users can still disable
this if they want.
Thanks @joshtriplett for the idea
Some more fixes in addition to 594c535ba2
* Some I noticed manually.
* Some were found by Topy (https://github.com/intgr/topy), either new
rules from Typo or new code in clap.
* `etc.` and `e.g.` rules were disabled during the last run.
