Switch to mutations instead of copy

[Maggi64]

Every function except `set' creates and returns a new array. I've done this to be consistent, but it doesn't always make sense.
To improve performance, we could switch to in-place mutations where it makes sense. These would be break changes.

Possible candidates:

• shuffle
• sort
• merge

Suggestion by acorn1010 #80

I am not sure yet. I would love to get some input on this.

[Camo30]

I prefer immutability over mutations. Many other libraries are using immutabilty so I think that's the way to go.