Integer mutators

[YVbakker]

This pull request will add the following mutators:

• Integer nullification mutator
• Integer negation mutator

Integer nullification mutator

Changes numeric literal expressions of type integer into 0. 0 is not mutated.

Examples:

• var x = 10 → var x = 0
• var x = -10 → var x = 0

Integer negation mutator

Changes numeric literal expressions of type integer into the negated form. 0 cannot be negated.

Examples:

• var x = 10 → var x = -10
• var x = -10 → var x = 10

closes #2849

[YVbakker]

thanks for this PR. I let you answer inline code questions. You need to adjust integration test results, since adding a mutator changes them

Thanks for your feedback, I'll get back to it this week