Add subject 'BehaviorSubject'

[utilForever]

This revisions includes:

• Add struct 'BehaviorSubject' (See http://reactivex.io/documentation/subject.html)
  • Implement local subject for 'BehaviorSubject'
  • Implement shared subject for 'BehaviorSubject'

p.s. As this is my first PR in this repo, i tried to inspire my implementation on struct 'Subject'.

[mergify]

Welcome to rxRust!
We're delighted to have you onboard <3

[M-Adoo]

Thanks, LGTM, and please update the CHANGELOG.md and MISSING_FEATURES.md

[mergify]

This pull request does not keep the same code style as this project. Could you fix it? 🙏 Switch to the nightly compiler (with rustup rustup default nightly) then run cargo fmt to get more details.

[codecov]

Codecov Report

Merging #153 (462bb12) into master (349e50b) will decrease coverage by 0.05%.
The diff coverage is 93.52%.

❗ Current head 462bb12 differs from pull request most recent head 50a5d46. Consider uploading reports for the commit 50a5d46 to get more accurate results

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   95.29%   95.23%   -0.06%     
==========================================
  Files          57       58       +1     
  Lines        4376     4515     +139     
==========================================
+ Hits         4170     4300     +130     
- Misses        206      215       +9     

Impacted Files	Coverage Δ
src/lib.rs	88.44% <ø> (ø)
src/subject/local_subject.rs	98.46% <93.33%> (-1.54%)	⬇️
src/subject/shared_subject.rs	97.36% <93.33%> (-2.64%)	⬇️
src/behavior_subject.rs	93.57% <93.57%> (ø)

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[utilForever]

@M-Adoo Please approve running workflows. ⛓️