Convert component values to JSON for testing

... rather than the other way around.
Work around Rust bugs (?) when matching extra::json::Json.
Catch up with spec changes, pass tinycss2 tests.
Remove explicit tests in favor of JSON tests.
Add consume_component_values_list()
Remove SyntaxError objects. "Bad" tokens are sufficient.
Factor out loading/parsing of JSON tests.
Rewrite the parser (non-tokenizer parts) to build again.
Test stand-alone declarations.