Remove uses of pin_project::project attribute

[taiki-e]

pin-project will deprecate the project attribute due to some unfixable
limitations.

Refs: taiki-e/pin-project#225

Although the deprecation of the project attribute has not been released yet, this PR is submitted in advance to avoid the CI from being broken by deprecated warnings.

[codecov]

Codecov Report

Merging #149 into master will increase coverage by 0.37%.
The diff coverage is 47.82%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   61.21%   61.59%   +0.37%     
==========================================
  Files          80       80              
  Lines        5036     5028       -8     
==========================================
+ Hits         3083     3097      +14     
+ Misses       1953     1931      -22     

Impacted Files	Coverage Δ
actix-service/benches/and_then.rs	0.00% <0.00%> (ø)
actix-service/src/apply_cfg.rs	0.00% <0.00%> (ø)
actix-service/src/and_then.rs	91.25% <66.66%> (+1.12%)	⬆️
actix-service/src/and_then_apply_fn.rs	89.61% <66.66%> (+1.14%)	⬆️
actix-service/src/then.rs	91.01% <66.66%> (+1.01%)	⬆️
actix-ioframe/src/service.rs	88.00% <100.00%> (+31.13%)	⬆️
actix-service/src/transform.rs	70.83% <100.00%> (+2.83%)	⬆️
actix-connect/tests/test_connect.rs	87.69% <0.00%> (-3.08%)	⬇️
router/src/resource.rs	73.45% <0.00%> (ø)
actix-ioframe/src/dispatcher.rs	55.14% <0.00%> (+0.93%)	⬆️
... and 1 more

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