Move Brisk.Layout to Brisk.Style and remove the Layout functor

[wokalski]

Those changes will make our lives much much simpler.

1. Remove this:
   https://github.com/briskml/brisk/blob/master/core/lib/Layout.re#L1

2. Move core/lib/Layout.re to renderer-macos/lib/Styles.re

3. Remove core