Blender Git Loki

BLI: cleanup StringRef and Span and improve parameter validation

Previously, methods like `Span.drop_front` would crash when more
elements would be dropped than are available. While this is most
efficient, it is not very practical in some use cases. Also other languages
silently clamp the index, so one can easily write wrong code accidentally.

Now, `Span.drop_front` and similar methods will only crash when n
is negative. Too large values will be clamped down to their maximum
possible value. While this is slightly less efficient, I did not have a case
where this actually mattered yet. If it does matter in the future, we can
add a separate `*_unchecked` method.

This should not change the behavior of existing code.