Exercises part 1

1. Declare and implement a function for filtering a vector similar to Data.List.filter.

2. Declare and implement a function for mapping a partial function over the values of a vector similar to Data.List.mapMaybe.

3. Declare and implement a function similar to Data.List.dropWhile for vectors. Use Data.DPair.Exists as your return type.

4. Repeat exercise 3 but return a proper dependent pair. Use the function from exercise 3 in your implementation.