his PR removes the items from the prelude that don't form a coherent
common vocabulary and adds the missing items that do.
Based on a comment at
<https://www.reddit.com/r/rust/comments/1cle18j/comment/l2uuuh7/>
BREAKING CHANGE:
The following items have been removed from the prelude:
- `style::Styled` - this trait is useful for widgets that want to
support the Stylize trait, but it adds complexity as widgets have two
`style` methods and a `set_style` method.
- `symbols::Marker` - this item is used by code that needs to draw to
the `Canvas` widget, but it's not a common item that would be used by
most users of the library.
- `terminal::{CompletedFrame, TerminalOptions, Viewport}` - these items
are rarely used by code that needs to interact with the terminal, and
they're generally only ever used once in any app.
The following items have been added to the prelude:
- `layout::{Position, Size}` - these items are used by code that needs
to interact with the layout system. These are newer items that were
added in the last few releases, which should be used more liberally.
7b45f74b71