Intern all strings of a mutation

[uhafner]

These strings are the same in a lot of mutations. Maybe we can switch to a TreeString in a future release of this model.

[codecov]

Codecov Report

Merging #14 (3640cfd) into main (f2f03a6) will decrease coverage by 0.13%.
The diff coverage is 46.15%.

@@             Coverage Diff              @@
##               main      #14      +/-   ##
============================================
- Coverage     85.55%   85.42%   -0.13%     
  Complexity      482      482              
============================================
  Files            23       23              
  Lines          1343     1345       +2     
  Branches        190      192       +2     
============================================
  Hits           1149     1149              
- Misses          148      150       +2     
  Partials         46       46              

Impacted Files	Coverage Δ
src/main/java/edu/hm/hafner/coverage/Mutation.java	77.52% <46.15%> (-1.79%)	⬇️

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