Width options (`width`, `minWidth`, `maxWidth`) options are now a part of the core column object.
useBlockLayout and useAbsoluteLayout hooks now use this new internalized information to implement
their layouts. Those examples have been updated. A virtualized-rows example has also been added to
show off how the useBlockLayout hook can be used to virtualize rows with react-window.
* useAbsoluteLayout: To build tables with divs
* Adding `placeholderOf` attribute to column
* Adding `useAbsoluteLayout` in index.js
* Adding `useAbsoluteLayout` example
* Adding `useAbsoluteLayout` in api docs
* Adding test for `useAbsoluteLayout` hook
useExpanded now uses a flat array of row path keys for tracking expanded state instead of nested
objects. This is both easier to use as a developer, but also enables expanding all rows or even
leaving nested rows in an expanded state, despite their parent rows' expanded state.
BREAKING CHANGE: See description
