This has a Data associated type, which is passed to parse(&str, data) -> Result<>

We'll use that Data to pass RsvgLength's auxiliary data for parsing.

Hopefully this way we can have a unified way to parse attribute values
from strings, without having exceptions just because:

  a) RsvgLength needs to know the direction upon parsing/construction;

  b) We can't impl FromStr for cairo::Matrix because they both come
  from extern traits.