Auto merge of #9929 - servo:cascade, r=pcwalton

Refactor the cascade

Converting the specified value of some properties into a computed value depends on the value of other properties. For example, the `em` unit of any length depends on the `font-size` property.

Previously, we would do a first pass over applicable declarations to build up a `values::computed::Context` struct with a number of fields for each such piece of data from other properties.

This simplies the struct by instead having it contain the set of computed values (for a given element) that is being populated and classify properties into "early" and "other", such that the only dependencies can be from "other" to "early". We iterate applicable_declarations twice, first cascading "early" properties then "other". Unfortunately, it’s not easy to check that this classification is correct.

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