Add layout utilities with tests.

Add layout utilities with tests.

Status: **Opened for visibility**
Reviewers: @kpeatt @jeffkamo @scalvert 

## Changes
- Add collection of layout utilities, including floats, float/margin containment (clearfix) and various formatting contexts.

## Notes
- Needs special attention on legacy platforms. In particular `u-bfc-noclip` needs testing on Android 2.3.