This adds a guide that explains how to implement a middleware from scratch without taking any shortcuts. It walks through implementing `Timeout` as it exists in Tower today.

The hope is that once users have read [the previous guide](https://tokio.rs/blog/2021-05-14-inventing-the-service-trait) followed by this one they should be fully equipped to implement their own middleware.

* REMOVE ME updates peak_wema test to pass

* adds pin_project_lite dependency

* uses pin_project_lite for load::Constant

* uses pin_project_lite for load::PencingRequestsDiscover

* uses pin_project_lite for load::PeakEwma

* uses pin_project_lite for load::Completion

* uses pin_project_lite for tests::support::IntoStream

Turns IntoStream into a regular struct because pin_project_lite does not and will support tuple structs.

https://github.com/taiki-e/pin-project-lite/blob/416be96f7777862c68b567c92a91887f69a8c2b3/src/lib.rs#L401-L408

* refactors opaque_future into a regular struct

This enables migration to pin_project_lite, which does not and will not support tuple structs
https://github.com/taiki-e/pin-project-lite/blob/416be96f7777862c68b567c92a91887f69a8c2b3/src/lib.rs#L401-L408

* migrates opaque_future to use pin_project_lite

* removes tuple variant from load_shed::ResponseState enum

* migrates load_shed::future to pin_project_lite

* removes tuple variant from filter::future::State

* migrates filter::future to pin_project_lite

Note: the doc comment on AsyncResponseFuture::service was also reduced to a regular comment.

This is a known limitation of pin_project_lite that the they have labeled as "help wanted".
taiki-e/pin-project-lite#3 (comment)

* migrates retry::Retry to pin_project_lite

* refactors retry::future::State to enable pin_project_lite

pin_project_lite has the current limitation of nto supporting doc comments
taiki-e/pin-project-lite#3 (comment)

pin_project_lite does not and will not support tuple variants
https://github.com/taiki-e/pin-project-lite/blob/416be96f7777862c68b567c92a91887f69a8c2b3/src/lib.rs#L401-L408

* migrates retry::future to pin_project_lite

* migrates spawn_ready::make to pin_project_lite

* refactors buffer::future::ResponseState to allow pin_project_lite

* migrates buffer::future to pin_project_lite

* refactors util::AndThenFuture to allow pin_project_lite

* migrates util::AndThenFuture to pin_project_lite

* migrates hedge::Future to pin_project_lite

* migrates hedge::select::ResponseFuture to pin_project_lite

* refactors hedge::delay enum for pin_project_lite

* refactors reconnect::future enum for pin_project_lite

* refactors oneshot::State enum for pin_project_lite

* migrates util::oneshot to pin_project_lite

* migrates reconnect::future to pin_project_lite

* migrates hedge::delay to pin_project_lite

* migrates hedge::latency to pin_project_lite

* migrates discover::list to pin_project_lite

* migrates timeout::future to pin_project_lite

* migrates balance::pool to pin_project_lite

* migrates balance::p2c::make to pin_project_lite

* migrates balance::p2c::service to pin_project_lite

* migrates call_all::ordered to pin_project_lite

* migrates call_all::common to pin_project_lite

* migrates call_all::unordered to pin_project_lite

* migrates util::optional::future to pin_project_lite

* migrates limit::concurrency::future to pin_project_lite

* migrates tower-balance example to pin_project_lite

* applies cargo fmt

* migrates tower-test to pin_project_lite

* fixes cargo hack check

peak_wma and pending_requests will now properly compile without the "discover" feature enabled.

* fixes lint rename warning on nightly

broken_intra_doc_links has been renamed to rustdoc::broken_intra_doc_links

* migrates buffer::Worker to pin_project_lite

pin_project_lite does support PinnedDrop
https://github.com/taiki-e/pin-project-lite/pull/25/files

However, it does not support generic trait bounds on the PinnedDrop impl.

To workaround this, I removed the T::Error bound from the Worker struct definition,
and moved `close_semaphore` to a a new impl without that trait bound.

* fixes abort_on_drop test

This test was also failing on master.

* applies cargo fmt

This one was missing.

Was the only combinator from `ServiceExt` that wasn't on
`ServiceBuilder` so now they match.

- Migrate to pin-project-lite ([#595])
- **builder**: Implement `Layer` for `ServiceBuilder` ([#600])
- **builder**: Add `ServiceBuilder::and_then` analogous to
  `ServiceExt::and_then` ([#601])

[#600]: #600
[#601]: #601
[#595]: #595
[pin-project-lite]: https://crates.io/crates/pin-project-lite