Add chop function for chopping iso Strings and Arrays

[dipinhora]

For context, my main use case for this functionality is efficient buffer manipulations (i.e. minimal or no copying). I had hoped to be able to accomplish this without modifying stdlib but the pointer manipulations required are only possible in builtin.

NOTE: This functionality is not safe until the bugs related to _alloc for String and Array trimming are fixed by #2336.

---

This commit adds a chop function to both String and Array
that works on an iso and chops it in two with the left half
being a new iso String/Array reusing the same pointer
and the right half being a trim_in_place version of the original
String/Array. Both are returned as iso and mutable and do not
share any memory. The operation does not allocate a new pointer
nor copy elements allowing for efficiency.

[dipinhora]

Updated to add chopping an already chopped string to the tests per #2340.

[SeanTAllen]

@dipinhora During sync we discussed ambiguity about what happens if the split point is outside the string. For example, a string or array of size 5 and a split point of 10. Can you add test(s) for that to demonstrate what happens as well as update the doc-strings to cover what the user should expect?

Other than that, this is looking good to merge.

[dipinhora]

@SeanTAllen Doh! Thanks to whoever pointed out that edge case.

I've added a case to both String and Array chop tests for that and fixed the logic to work correctly also.

[SeanTAllen]

@dipinhora I think it was @Praetonus but I don't remember.