New mutation type: fiddle with switch statement cases

[ZevEisenberg]

Before:

switch myEnum {
    case .a: doFunctionA()
    case .b: doFunctionB()
}

After:

switch myEnum {
    case .a: doFunctionB()
    case .b: doFunctionA()
}

Makes the most sense for enums without associated values. Associated values might be interesting to think about, but probably as a separate feature.