More nesting plumbing. Still does nothing because we don't parse the
nested rules.

Should be trivial to prove this patch doesn't change any behavior so
far, but I want to land it on its own because it can have performance
implications.

This follows the pattern of what we do with other rules like layers and
container conditions, that is, keep the ancestor selectors in a stack,
and poke at the last one in order to replace the ancestor.

This changes the behavior of replace_parent_selector as with the newer
version of the spec, stuff like:

  div {
    .foo {
      stuff
    }
  }

Should work as `div .foo`. A test is added for this case.

Differential Revision: https://phabricator.services.mozilla.com/D176560

…s along with declaration

No implementation just yet (the default QualifiedRuleParser
implementation just rejects stuff), but this is plumbing that I'd rather
get reviewed separately.

Differential Revision: https://phabricator.services.mozilla.com/D176686

NaN and infinity percentages are now serialized as expected.

Also added some new WPT tests as percentages were previously untested
and added some spec comments to previous NaN/inf serialization code.

Differential Revision: https://phabricator.services.mozilla.com/D176726

Refactored ImportLayer into an enum instead of a struct and using
Option everywhere.

Differential Revision: https://phabricator.services.mozilla.com/D176793

Although we store animation and transition style values in StyleUIReset and
define their properties in longhands/ui.mako.rs, but we may move them in
the future if this style struct becomes too large. So let's move the
definition of their values to an independent module, animation, so we
don't have to worry about this again.

This patch doesn't change any other things. Only move code.

Differential Revision: https://phabricator.services.mozilla.com/D173903

Support view() notation for animation-timeline:
  `<view()> = view( [ <axis> || <'view-timeline-inset'> ]? )`

We move AnimationTimeline and its related types into the generics folder,
and define two new structs for scroll() and view().

Note:
1. The syntax of scroll() doesn't match the current version of the spec.
    I will update it in Bug 1814444.
2. We will handle the creation/usage of the Anonymous View Progress Timelines
    in the next patch.

Differential Revision: https://phabricator.services.mozilla.com/D173904

Sum nodes would use mul_by to negate nodes to do subtraction, but some
nodes are not distributive.  This patch adds a negate node, so that the
operations inside these negate nodes can be resolved first and then the
"subtraction" can be applied.

Differential Revision: https://phabricator.services.mozilla.com/D172941