Once this is done we should be able to call it a day and call this feature complete.

We'll use CalcNode as the specified value representation for <length> and
<length-percentage> values, so they'll have to implement ToCss.

There's one minor issue (two calls to to_css() instead of to_css_impl() which
are addressed later in the series).

We'll have different leaf nodes as we progress in the value computation stage.

Depends on D63395

This is the meat of the patch. There are a couple improvements done in a couple
later patches which should hopefully be straight-forward.

Depends on D63396

Depends on D63397

So as to avoid allocating an intermediate tree in Rust to resolve
<length-percentage> values.

Depends on D63398

We were serializing calc(10% + 4px) as calc(10% + calc(4px)).

Depends on D63399

We never fast-reject numbers (because they could be part of a product). Without
this refactoring we'd accept stuff like calc(10) and crash during the evaluation
for obvious reasons.

Depends on D63400

See https://github.com/web-platform-tests/wpt/issues/21599 for the specified
value serialization failures.

Depends on D63401

https://hg.mozilla.org/mozilla-central/rev/5de6753fff0d
https://hg.mozilla.org/mozilla-central/rev/15bce74b1954
https://hg.mozilla.org/mozilla-central/rev/b9ef57ebff2a
https://hg.mozilla.org/mozilla-central/rev/be7e5e643ef7
https://hg.mozilla.org/mozilla-central/rev/66a5e8c28803
https://hg.mozilla.org/mozilla-central/rev/87e569cafaaa
https://hg.mozilla.org/mozilla-central/rev/0cb57e6d574f
https://hg.mozilla.org/mozilla-central/rev/55f06c70f4e5