Bugfix: Partial string functions are incorrect.

Bugfix: Partial string functions are incorrect.

The tests exposed these problems.

The *_assure() functions are operating as if they are *_assure_nulless().
Remove the NULL checks that should not be there and fix the documentation comments.

The *_assure() functions are not considering the following:
1) The case where range.stop is >= source.used.
2) The fact that range.stop is inclusive and should not be directly used in the same way that *.used is used.

In the case of (1), if the stop range result in it overflowing past the actual length, the previous code results in invalid reads.
In the case of (2), the stop.range needs to instead be stop.range + 1.