Cleanup

[aleksandy]

• use toAray(IntFunction) instead of toArray(T[])
• optimize JavaExtensions#remove()
• simplify JavaExtensions#pad()
• simplify JavaExtensions#join()
• use primitives instead of wrappers
• use pattern matching instead of explicit cast
• use String.valueOf() and toString() for the objects stringification instead of concatenation with empty string

[aleksandy]

@xael-fry, @asolntsev, could you review my PR?