Report at-rule sizes

[bartveneman]

• [ ] At-rule Ruleset count
• [ ] At-rule declaration count
• [ ] At-rule selector count
• [ ] At-rule descriptor count
• [ ] At-rule total size (declarations + selectors)
• [ ] Aggregate sizes too? Like avg. declarations/rules/selectors per at-rule

If we ditch CSSTree's walker and use our own walking implementation instead, we can also measure sizes quite well.