fix(instrumenter): switch case mutant placer

[nicojs]

Add a MutantPlacer for SwitchCase nodes. It places the mutants in the consequence part of the switch case. This can dramatically improve the performance of instrumenting large switch-case statements.

This:

switch(foo){
  case 'bar':
    console.log('bar');
    break;
  case 'baz':
    console.log('baz');
    break;

}

Is now instrumented as:

switch (foo) {
  case stryMutAct_9fa48(1) ? \\"\\" : (stryCov_9fa48(1), 'bar'):
    if (stryMutAct_9fa48(0)) {} else {
      stryCov_9fa48(0);
      console.log(stryMutAct_9fa48(2) ? \\"\\" : (stryCov_9fa48(2), 'bar'));
      break;
    }

  case stryMutAct_9fa48(4) ? \\"\\" : (stryCov_9fa48(4), 'baz'):
    if (stryMutAct_9fa48(3)) {} else {
      stryCov_9fa48(3);
      console.log(stryMutAct_9fa48(5) ? \\"\\" : (stryCov_9fa48(5), 'baz'));
      break;
    }
}

While previously the entire SwitchCaseExpression was duplicated for each empty switch case mutation.

Fixes #2401

[nicojs]

@brodybits

22:27:14 (25864) INFO InputFileResolver Found 1 of 6726 file(s) to be mutated.
22:27:16 (25864) INFO Instrumenter Instrumented 1 source file(s) with 6876 mutant(s)

Instrumenting printer-estree.js now takes a fraction of a second 👍

[brodybits]

🎉 I hope to try it soon!

[nicojs]

Feel free to review the code if you want 😎

[brodybits]

Unfortunately I still do not understand the mutation switching well enough to comment on the changes or on the test cases. Thanks for the investigation and work on this.