Add Array#insert_all

[summer-alice]

This adds a method to Array that allows you to insert all elements from one array into another at a given index.

While this is already possible in Crystal using range assignment, the syntax and side effects may not be immediately obvious. Achieving the desired behaviour can also be error-prone as it depends on the number of dots used.

arr = [1, 2, 6]
# arr[2..2] = [3, 4, 5]   # => [1, 2, 3, 4, 5]
arr[2...2] = [3, 4, 5]    # => [1, 2, 3, 4, 5, 6]

The new method would simplify it to:

arr = [1, 2, 6]
arr.insert_all(2, [3, 4, 5]) # => [1, 2, 3, 4, 5, 6]

(For further reference I did create a post on the forum)