Change all is_* functions into a single get_type that return an enum

[MeirShpilraien]

Follow the conversation on: RedisGears/RedisGears#876 (comment).
We should change all is_* type of functions into a single get_type() function that will return an enum and will allow us to check the type using a match operation. This will help to avoid issues like forgetting to check for a specific type and it is more of the rust way of handling types.