- merge `Style` and `StyleDiff` together. `Style` now is used to activate or deactivate certain
style rules not to overidden all of them.
- update all impacted widgets, examples and tests.
* Remove custom Debug implementation of Buffer
* Add `TestBackend::assert_buffer` to compare buffers in integration tests. When
the assertion fails, the output now show the list of differences in addition
of the views of the computed and expected buffers. This effectively replaces
the table of debug code for colors and modifiers as it is easier to read.