Add support for Swift 5.5 and async/await

[ptoffy]

Updated swift-tools-version to 5.5 and added async/await support to the project.
Removed EventLoopFuture tests as they are redundant when using async/await.

[0xTim]

@ptoffy can you merge in the latest changes from main - that should trigger CI

[ptoffy]

@0xTim I think the tests are failing because the installed version of swift on CI is 5.2 and 5.4

[0xTim]

@ptoffy yeah, now the CI is in your fork you should be able to change swift:5.2-xenial to swift:5.5 etc

[codecov]

Codecov Report

Merging #21 (6b06aff) into main (f8d4517) will increase coverage by 13.27%.
The diff coverage is 97.43%.

@@             Coverage Diff             @@
##             main      #21       +/-   ##
===========================================
+ Coverage   83.33%   96.61%   +13.27%     
===========================================
  Files           2        2               
  Lines          66       59        -7     
===========================================
+ Hits           55       57        +2     
+ Misses         11        2        -9     

Impacted Files	Coverage Δ
...rces/LeafErrorMiddleware/LeafErrorMiddleware.swift	95.91% <97.29%> (-1.91%)	⬇️
...ddleware/LeafErrorMiddlewareDefaultGenerator.swift	100.00% <100.00%> (+50.00%)	⬆️

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[ptoffy]

@0xTim have you had any time to review this?

[0xTim]

This is great thanks! There's just a couple of missing tests and we're good to go!

[0xTim]

Thanks!