Range#step: restore legacy behavior for String ranges

[zverok]

See discussion in https://bugs.ruby-lang.org/issues/18368#note-34

Of options I’ve named I implemented support for String ranges for both old and new behavior:

# Old behavior with integer steps and #succ
('a'..).step(2).take(3)
#=> ["a", "c", "e"]
('a'..).step.take(3)
#=> ["a", "b", "c"]

# New behavior with string step and #+
('a'..).step('b').take(3)
#=> ["a", "ab", "abb"]

[knu]

Thank you. The functions step_i_iter() and step_i() are exactly what existed before the change, so this PR accurately tries to resurrect the lost compatibility.

You could do the same for symbols. What do you think?

[zverok]

You could do the same for symbols. What do you think?

@knu, sorry, it took me some time to find a minute to work on this.

Here we go: #11573