Merge cssparser.rs into cssparser.rc
Remove the quirks mode flag.

It’s part of tree construction, not the tokenization.
Add an opaque Tokenizer object with methods.
Handle parse errors by tokens.

* Remove the error array
* Add TokenResult, which is a Token with an optional error
* Update parts of the tokenizer to use TokenResult.

Each token has zero or one parse error.
Separate public and non-public tokenizer API
Rename tokenizer.rs to tokens.rs
Add data structures for tree construction.