Muter supports inverting compound assignment operators

[SeanROlszewski]

see how stryker defines it for an example: https://github.com/stryker-mutator/stryker-handbook/blob/master/mutator-types.md#assignment-expression

[SeanROlszewski]

this is de-prioritized because it involves needing to build a system that does type inference, which we plan on doing but are not focused on now