Add string pruning function

[totakke]

Adds proton.string namespace and proton.string/prune function. prune is useful to shorten a long text with indicating the text is omitted.

[codecov]

Codecov Report

Merging #5 into master will increase coverage by 0.88%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   91.33%   92.21%   +0.88%     
==========================================
  Files           4        5       +1     
  Lines         150      167      +17     
  Branches        3        3              
==========================================
+ Hits          137      154      +17     
  Misses         10       10              
  Partials        3        3

Impacted Files	Coverage Δ
src/cljc/proton/string.cljc	100% <100%> (ø)

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[federkasten]

Good enhancement. Thank you.

I would like to expand string utilities much more.