Improve text circle-fitting.

[nathanhammond]

The algorithm for text circle-fitting is currently expensive in multiple ways:

• Requires reviewing every permutation of possible line-breaks.
• Permutations include duplication.
• Lots of likely unnecessary intermediate values.
• Interleaved DOM reads and writes triggering layout.
• Not easy to understand what it's doing.
• Tightly coupled with where it is used.

It works, but is worth refactoring into an easily-usable plugin or three:

• circle-fitter
• textblock-permutations
• baseline-measure