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.