Support awaiting optional Promises

[WilsonGramer]

This PR adds support for awaiting optional promises, which allows for optional chaining inside an await function, like so:

try await(self.foo?.bar())

If the promise is nil, await immediately returns nil and does nothing more; otherwise it executes the promise as normal and returns an Optional wrapping the awaited value. This is implemented for both Promises and Guarantees.

This should resolve #19 — please let me know what you think, and thank you for providing this library!

[codecov[bot]]

Codecov Report

Merging #49 into master will decrease coverage by 2.35%. The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage     100%   97.64%   -2.36%     
==========================================
  Files           4        4              
  Lines          75       85      +10     
==========================================
+ Hits           75       83       +8     
- Misses          0        2       +2

Impacted Files	Coverage Δ
Sources/AwaitKit/AwaitKit.swift	90% <80%> (-10%)	:arrow_down:

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[WilsonGramer]

Hey @yannickl, what's the status on this PR? Thanks 😄