Add `Type::getKeysArray()` and `Type::getValuesArray()`

[herndlm]

low-hanging fruit cleanup / improvement :)

• fixes ArrayType::getKeysArray() (by using getIterableKeyType() mixed keys are correctly transformed into benevolent unions)
• simplifies list type creation for array_is_list
• gives us support for constant array unions for free (instanceof ConstantArrayType be gone! one of the main reasons for all this refactoring)