Add `Using` operator and tests.

[danielt1263]

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Adding the Using "operator" which I use quite a bit when working with RxSwift.

[danielt1263]

Conflicts fixed. The operator is ready to be merged in with the next dot release.

[codecov[bot]]

Codecov Report

Merging #111 (363eeb7) into main (36e8564) will increase coverage by 0.06%. The diff coverage is 98.64%.

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   95.43%   95.49%   +0.06%     
==========================================
  Files          66       68       +2     
  Lines        3725     3799      +74     
==========================================
+ Hits         3555     3628      +73     
- Misses        170      171       +1     

Impacted Files	Coverage Δ
Tests/UsingTests.swift	98.30% <98.30%> (ø)
Sources/Operators/Using.swift	100.00% <100.00%> (ø)

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