layout: Rewrite whitespace stripping.

This patch makes Servo unconditionally strip whitespace before text run
scanning (assuming that the `white-space` property allows it). Whitespace
stripping during reflow is now only used for handling whitespace at the ends of
lines; reflow now never attempts to handle ignorable whitespace.

Many CSS tests pass now. There are some new failures, however.

The following reference tests now fail due to a pre-existing bug whereby
whitespace is used to calculate the position of inline hypothetical boxes for
elements with `display: inline; position: absolute`:

* `absolute-replaced-height-036.htm`
* `vertical-align-sub-001.htm`
* `vertical-align-super-001.htm`

The following reference tests fail due to a pre-existing bug whereby we don't
handle `font-size: 0` properly in inline reflow:

* `font-size-zero-1.htm`
* `font-size-zero-2.htm`

The following reference test fails due to the fact that it relied on our
incorrect insertion of whitespace to make room for the black background:

* `inline-formatting-context-007.htm`

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