Revert pin project lite

[seanmonstar]

It seems that the pin-project-lite isn't noticing the cfg(feature = "..") attributes on enum variants or fields, and also our CI didn't notice the failure of combining --features http1,client,server.

I'm reverting this change since it's the fastest way for me to unbreak and get a new release out.

Closes #2421

cc @jplatte @taiki-e

[taiki-e]

Oh, both seem to be related to the lack of documentation of crates I'm developing... (cargo-hack actually documents it, but that the current readme just lists the options and it is not very easy to read.)

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It seems that the pin-project-lite isn't noticing the cfg(feature = "..") attributes on enum variants or fields

I think this is the limitation I mentioned in #2388 (comment).

pin-project-lite#3 is about attributes that apply only to the original struct/enum.
Proper handling of cfg/cfg_attr on fields and variants requires a more complex approach than the fix for pin-project-lite#3 (it needs to be aware of the kind of attribute and propagate some of it to projected type). Even pin-project could not find the correct solution other than "leave to the compiler". And it is impossible to use the same trick in pin-project-lite.

Perhaps a similar workaround as mentioned in that comment will work.

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also our CI didn't notice the failure of combining --features http1,client,server.

This can be fixed by using cargo hack --feature-powerset (all combination of features) instead of cargo hack --each-feature (each feature) in CI.
However, if we use --feature-powerset, there will be more than 700 feature combinations, so we may need to use --depth (specify a max number of simultaneous feature flags of --feature-powerset) flag to limit the number of combinations.

Filed #2423 for this.