Previously, some StyleValues created a large number of intermediate
strings during serialization. Passing a StringBUilder into the
serialization function allows us to avoid a large number of these
unnecessary allocations.
`EdgeStyleValues` which consist of an offset of a `calc()`s which
resolves to 50% should be considered "centered" for
`SerializationMode::ResolvedValue` for the purpose of omitting the
position value from gradient serialization.
As well as being required to implement absolutization this also means we
now bypass a limitation with `LengthPercentage` where we would always
use `SerializationMode::Normal` for the constituent lengths which gains
us some WPT passes
The CSSNumericType defined in the spec is a simple dictionary which is
only used for OM purposes. This NumericType class is used internally
and matches the more abstract definition of a "type".
Calc simplification (which I'm working towards) involves repeatedly
deriving a new calculation tree from an existing one, and in many
cases, either the whole result or a portion of it will be identical to
that of the original. Using RefPtr lets us avoid making unnecessary
copies. As a bonus it will also make it easier to return either `this`
or a new node.
In future we could also cache commonly-used nodes, similar to how we do
so for 1px and 0px LengthStyleValues and various keywords.
This reverts commit 76daba3069.
We're going to need separate types for the JS-exposed style values, so
it doesn't make sense for us to match their names with our internal
types.
