Make TextPiece hierarchical, containing either text or a slice of other TextPieces.
Merge RichText functionality into TextPiece.
gofmt
Make TextPiece fields private where their values may need to be calculated recursively or where they are purely an implementation detail.
Remove Tokens as they were being conflated with words.
Set TextPiece.NoBreak from nobreak option in constructor.
Merge TextLine functionality into TextPiece.
Remove now-redundant RichText and TextLine.
Rename TextPiece to RichText.
Update benchmark times.