layout: Implement `overflow-wrap`/`word-wrap` per CSS-TEXT § 6.2.

This property is used by approximately 55% of page loads.

To implement the line breaking behavior, the "breaking strategy" has
been cleaned up and abstracted. This should allow us to easily support
other similar properties in the future, such as `text-overflow` and
`word-break`.
auto merge of #4361 : pcwalton/servo/overflow-wrap, r=glennw

This property is used by approximately 55% of page loads.

To implement the line breaking behavior, the "breaking strategy" has
been cleaned up and abstracted. This should allow us to easily support
other similar properties in the future, such as `text-overflow` and
`word-break`.

r? @glennw