String: rename split without separator to words

[makenowjust]

See #4644 (comment)

Now, String#split without separator is deprecated (breaking change).

[straight-shoota]

I think it could be argued to add String#words() besides String.split() handling leading and trailing whitespace differently. But removing String.split() makes no sense to me.

[need47]

I think we just need a clear documentation on this, and that's enough.

[makenowjust]

@straight-shoota We can use s.split(/\s+/) explicitly if need such a result. Of course, there is performance issue, however I don't know when such a behavior is needed.

[makenowjust]

To fit String#split without separator semantics, Ruby decided to remove leading and trailing empty strings from other split result. However Crystal decided to keep them. At that time, String#split without separator is broken.

[RX14]

I believe String#split should behave exactly the same as String#split(' ') (except for splitting on all unicode whitespace (current is ascii-only)). Adding a String#words with the old implementation would be a good idea.

[makenowjust]

We take care of String#split's default separator. For example, in JavaScript String.prototype.split makes an array containing the entire string if separator is not given. You may believe String#split's default separator is whitespace and it is the only answer without reason, but I am not believer. I think String#split should be given separator explicitly.

However, in fact, strangely, current Crystal behavior is almost same as Python's one (Python does not support empty string separator, but others are same). As @need47 said, we may have to write a clear documentation like Python. I agree with it if prefer simple solution.

[bew]

For others not following everything, this PR has been closed because this comment.