New mutant operators

[DurieuxPol]

Added several mutant operators that were missing and could be interesting:

• MTAssignmentNullifierOperator which transforms the value of an assignment to nil: variable := value -> variable := nil
• MTReplaceWhileTrueWithWhileFalseMutantOperator and MTReplaceWhileFalseWithWhileTrueMutantOperator
• MTReplaceWhileTrueReceiverWithTrueOperator and MTReplaceWhileTrueReceiverWithFalseOperator
• MTReplaceWhileFalseReceiverWithTrueOperator and MTReplaceWhileFalseReceiverWithFalseOperator

I found the assignment operator while experimenting a bit. With the current operators, especially MTEmptyMethodOperator, there was never a method without mutation because this operator could always be applied.
So I tried to get methods without mutation without taking into account this operator, and I found several methods with only assignments, like the initialize methods.
As for the operators on whileTrue: and whileFalse:, I was just looking at the ones on ifTrue: and ifFalse: and noticed that there wasn't any operator for their while counterparts.

[coveralls]

Pull Request Test Coverage Report for Build 7728215628

• 94 of 288 (32.64%) changed or added relevant lines in 17 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.8%) to 58.108%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/MuTalk-Tests/MTReplaceWhileFalseReceiverWithFalseOperatorTest.class.st	4	31	12.9%
src/MuTalk-Tests/MTReplaceWhileFalseReceiverWithTrueOperatorTest.class.st	4	31	12.9%
src/MuTalk-Tests/MTReplaceWhileFalseWithWhileTrueMutantOperatorTest.class.st	4	31	12.9%
src/MuTalk-Tests/MTReplaceWhileTrueReceiverWithFalseOperatorTest.class.st	4	31	12.9%
src/MuTalk-Tests/MTReplaceWhileTrueReceiverWithTrueOperatorTest.class.st	4	31	12.9%
src/MuTalk-Tests/MTReplaceWhileTrueWithWhileFalseMutantOperatorTest.class.st	4	31	12.9%
src/MuTalk-Tests/MTAssignmentNullifierOperatorTest.class.st	4	36	11.11%

Totals
Change from base Build 7653796460:	-1.8%
Covered Lines:	4293
Relevant Lines:	7388

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💛 - Coveralls

[guillep]

Thanks @DurieuxPol !

Could you write in the issue description how did you find these missing operators?

[guillep]

Good catch of dead code :)