Add a coercing function of boolean

[totakke]

Adds as-boolean. This function is different from Boolean/parseBoolean in that it

• also supports "Yes" and "No"
• returns nil if an illegal string is passed (Boolean/parseBoolean returns false in that case).

I feel this behavior is strict and useful.

[codecov]

Codecov Report

Merging #3 into master will increase coverage by 0.2%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master       #3     +/-   ##
=========================================
+ Coverage   90.51%   90.71%   +0.2%     
=========================================
  Files           4        4             
  Lines         137      140      +3     
  Branches        3        3             
=========================================
+ Hits          124      127      +3     
  Misses         10       10             
  Partials        3        3

Impacted Files	Coverage Δ
src/cljc/proton/core.cljc	91.04% <100%> (+0.41%)	⬆️

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[federkasten]

Thank you :)