Add mutator to remove shared cases

[Khartir]

This PR:

• Adds a new mutator to remove cases from switch-statments that share a body
• Covered by tests
• Doc PR: Add description for SharedCaseRemoval mutator site#173

Closes #1290

Code like this:

switch ($value) {
  case 'a':
    doSomething();
    break;
  case 'b':
  case 'c':
  default:
    doSomethingElse();
    break;
}

Creates the following mutants:

switch ($value) {
  case 'a':
    doSomething();
    break;
-  case 'b':
  case 'c':
  default:
    doSomethingElse();
    break;
}

switch ($value) {
  case 'a':
    doSomething();
    break;
  case 'b':
-  case 'c':
  default:
    doSomethingElse();
    break;
}

switch ($value) {
  case 'a':
    doSomething();
    break;
  case 'b':
  case 'c':
-  default:
    doSomethingElse();
    break;
}

[maks-rafalko]

Thanks for working on this new Mutator! Looks good to me except minor comments above.

Question: why do we remove only shared cases? Why not to remove case by case?

[Khartir]

@maks-rafalko I only removed shared cases because there is no way to see they are missing tests by looking at the code-coverage.
Removing a "normal" case would be like removing an entire if. I assumed that this would be better detected by the existing mutators mutating the code inside the case.
Which brings up the edge case of

case 'a':
  break;

I'm unsure about this. If you want this to remove all cases (including their body) I can do so. There should then probably also be a mutator to remove if and else.

What do you think?

Regarding the suggested changes: I'll probably look into it over the weekend.

[maks-rafalko]

I'm ok with removing only shared cases :) Thanks, looking forward to getting it merged!

[maks-rafalko]

Thank you @Khartir!