Begin speculative CSS lexer.
Move `list_to_json` to "to_json.rs"

It is useful beyond just the tests in `css_lex`.
Make lexer structures cloneable.
Initial (working) version of specfold CSS lexing

It has a lot of magic numbers, but it works! Next is to make it more
flexible.
Remove JSON output and fix prediction failure

JSON is no longer printed and prediction failures are properly handled.