For example, this:
```css
.foo {
color: red;
&:hover {
color: green;
}
}
```
now has the same effect as this:
```css
.foo {
color: red;
}
.foo:hover {
color: green;
}
```
CSSStyleRule now has "absolutized selectors", which are its selectors
with any `&`s resolved. We use these instead of the "real" selectors
when matching them, meaning the style computer doesn't have to know or
care about where the selector appears in the CSS document.
