#1582 Optimize `Document::SetStyles()` and `Document::SetStyleFor()`

[Zufu Liu]

Using idea of SplitView, Document::SetStyles() and Document::SetStyleFor() can be optimized to update style on each segment, this eliminated four boundary check (style.ValueAt() + style.SetValueAt()) for each byte. when precise range for ModificationFlags::ChangeStyle notification is not needed (like Document::SetStyleFor()), memcmp, memcpy, and memset can be used to set new styles, attached patch does this (use single method for CellBuffer to reduce code size, Document::SetStyleFor() and Document::SetStyles() can be changed to call a new method to reduce duplication).

[Neil Hodgson]

This appears to produce a larger range in the ChangeStyle notification that could require more work for the application to process.

[Zufu Liu]

It's same as Document::SetStyleFor(), which report entail range as changed.