Auto merge of servo#328 - tiaanl:new-parsing, r=emilio

Combine parsing into legacy and new parsing.

Combine the rules of legacy and new parsing into single functions.  This allows for a much clearer separation of which keywords support which syntax.  RGB is still special, because it the components **must** be of the same type (numbers or percentages).  Most of the color functions are using the new syntax, but some special notes:

- `RGB` uses it's own parser as mentioned.
- `HSL` supports both legacy and new syntax.
- `HWB` only supports new suntax.
- _ the rest supports the new syntax only.

These changes are also in preparation of introducing the `none` keyword where it is much more important to know which syntax is being used, as legacy syntax does **not** support the `none` keyword.

Author: bors-servo