Add `Set#select!`, `Set#reject!`, and `Set#map!`

[potomak]

I'm also adding:

• Set#keep_if, used in the implementation of Set#select!
• Set#delete_if, used in the implementation of Set#reject!
• Set#filter!, an alias for #select!

Closes #9976

[HertzDevil]

Please don't add the method name aliases. The Crystal standard library no longer features them.

The methods #select! and #reject! never return nil in Crystal. You could see this being the case in Array and Hash.