Migrate to pin-project-lite

[davidpdrsn]

Tower recently migrated to pin-project-lite (tower-rs/tower#595) so it makes sense to do the same for tower-http. It should be a fairly mechanical operation. Off the top of my head tower-http isn't using any of pin-projects more advanced features.

[tesaguri]

Migrating compression::Body and dexompression::Body would be difficult because their inner enums have #[cfg(...)] attributes on its variants, which pin-project-lite does not support yet (taiki-e/pin-project-lite#3). There is a workaround (taiki-e/pin-project-lite#3 (comment)) like the one I originally used in #41, though it would introduce a huge macro.

[jplatte]

Working on this.