Major pain point is that I had to write the bitfield stuff manually, but that
should be resolved soon again.
Now we generate proper layout for _every_ struct, including field offsets \o/.
* `LonghandId` and `ShorthandId` are C-like enums
* `Atom` is used for the name of custom properties.
* `PropertyDeclarationId` is the identifier for `PropertyDeclaration`,
after parsing and shorthand expansion. (Longhand or custom property.)
* `PropertyId` represents any CSS property, e.g. in CSSOM.
(Longhand, shorthand, or custom.)
Using these instead of strings avoids some memory allocations and copies.
