rework Rejection system

[seanmonstar]

The Rejection type can now nest and combine arbitrary rejections,
so it is no longer bound to a small set of meanings. The ranking of
status codes is still used to determine which rejection gets priority.

A different priority can be implemented by handling rejections with
a Filter::recover, and searching for causes in order via
Rejection::find_cause.

• Adds warp::reject::custom() to create a Rejection with
  any Into<Box<std::error::Error>>. These rejections should be
  handled with an eventual Filter::recover. Any unhandled
  custom rejections are considered a server error.
• Deprecates Rejection::with. Use custom rejections instead.
• Deprecates Rejection::into_cause, as it can no longer work. Always
  returns Err(Rejection).
• Deprecates Rejection::json, since the format needed is too generic.
  The errors.rs example shows how to send custom JSON when recovering
  from rejections.
• Deprecates warp::reject(), since it current signals a 400 Bad Request, but in newer versions, it will signal 404 Not Found.
  It's deprecated simply to warn that the semantics are changing,
  but the function won't actually go away.
• Deprecates reject::bad_request(), reject::forbidden(), and
  reject::server_error(). Uses custom rejections instead.

[seanmonstar]

Having thought on this over the week, I've decided to remove the reject::status(code) part. The purpose of the rejections isn't to provide a mirror-ed Reply interface. It's simply a specialized error system to be used in warp. To customize how a rejection should be rendered, the Filter::recover combinator can be used to inspect rejections and create a Reply.

[seanmonstar]

Here's the updated change:

---

The Rejection type can now nest and combine arbitrary rejections,
so it is no longer bound to a small set of meanings. The ranking of
status codes is still used to determine which rejection gets priority.

A different priority can be implemented by handling rejections with
a Filter::recover, and searching for causes in order via
Rejection::find_cause.

• Adds warp::reject::custom() to create a Rejection with
  any Into<Box<std::error::Error>>. These rejections should be
  handled with an eventual Filter::recover. Any unhandled
  custom rejections are considered a server error.
• Deprecates Rejection::with. Use custom rejections instead.
• Deprecates Rejection::into_cause, as it can no longer work. Always
  returns Err(Rejection).
• Deprecates Rejection::json, since the format needed is too generic.
  The errors.rs example shows how to send custom JSON when recovering
  from rejections.
• Deprecates warp::reject(), since it current signals a 400 Bad Request, but in newer versions, it will signal 404 Not Found.
  It's deprecated simply to warn that the semantics are changing,
  but the function won't actually go away.
• Deprecates reject::bad_request(), reject::forbidden(), and
  reject::server_error(). Uses custom rejections instead.