A version of split that returns an array of substrings

[ViralBShah]

I often find myself wanting an array of SubStrings rather than Strings as the output of split, in order to save memory and reduce GC pressure.

It would prefer to return an array of SubStrings by default, but I believe SubString indexing is slow (unsafe_view can help for now), and also any modifications to the SubStrings will get reflected in the original String (which I think is ok).

At the very least, I would propose to have splitsub.

[JeffBezanson]

Modifying strings isn't really an issue since we don't support it.

[JeffBezanson]

Could also fix #1250. For a string of type T, split giving an array of SubString{T} seems totally reasonable to me. @StefanKarpinski

[ViralBShah]

Of course, just realized that we changed String to immutable. In that case, this seems even more reasonable to do. SubString indexing can be improved upon, I am sure.

[ViralBShah]

Given the matchall implementation that @dcjones just did with SubStrings, and also the recent performance improvement in substring indexing, does it make sense to make split also return SubStrings? This is usually what I want.

For example, we could then use it in dlmread and simplify the code in there. Cc: @tanmaykm

[simonster]

Is this fixed by #5451 / 1234340?

[quinnj]

Bump, as @simonster, noted, I believe this is fixed.