VALUES-4 defines the internal representation of `calc()` as a tree of
calculation nodes. ( https://www.w3.org/TR/css-values-4/#calc-internal )
VALUES-3 lacked any definition here, so we had our own ad-hoc
implementation based around the spec grammar. This commit replaces that
with CalculationNodes representing each possible node in the tree.
There are no intended functional changes, though we do now support
nested calc() which previously did not work. For example:
`width: calc( 42 * calc(3 + 7) );`
I have added an example of this to our test page.
A couple of the layout tests that used `calc()` now return values that
are 0.5px different from before. There's no visual difference, so I
have updated the tests to use the new results.
Level 4 drops the limitations of what types can be a denominator, which
means `<calc-number-sum>`, `<calc-number-product>` and
`<calc-number-value>` all go away.
