Add Guarantees Support

[djtech42]

allow await(guarantee)
(currently guarantees do not work here as they are not Promises)

[codecov]

Codecov Report

Merging #39 into master will decrease coverage by 2.56%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   69.23%   66.66%   -2.57%     
==========================================
  Files           4        4              
  Lines          26       36      +10     
==========================================
+ Hits           18       24       +6     
- Misses          8       12       +4

Impacted Files	Coverage Δ
Sources/AwaitKit/AwaitKit.swift	0% <0%> (ø)	⬆️
Sources/AwaitKit/DispatchQueue+Await.swift	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #39 into master will increase coverage by 1.35%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   69.23%   70.58%   +1.35%     
==========================================
  Files           4        4              
  Lines          26       34       +8     
==========================================
+ Hits           18       24       +6     
- Misses          8       10       +2

Impacted Files	Coverage Δ
Sources/AwaitKit/AwaitKit.swift	0% <0%> (ø)	⬆️
Sources/AwaitKit/DispatchQueue+Await.swift	100% <100%> (ø)	⬆️

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[yannickl]

Thank you very much for your contribution!