Revisit parse_options and parse_error

[tgockel]

The current parse_options (and the associated parse_error) has a few things that are no longer relevant.

• failure_mode: A parse error should not have a failure mode of any significance. Skipping over a token is dangerous at best. However, an extraction error can logically have on_error::fail_immediately and on_error::collect_all options (on_error::ignore seems deeply misguided)
• max_failures: No longer relevant.
• string_encoding: Remove encoding::cesu8
• number_encoding: Remove entirely -- allowing leading zeros is confusing in the first place
• comma_policy: Remove entirely -- treating trailing commas as whitespace is dangerous and slows the parser down
• max_structure_depth: Keep.
• require_document: Keep.
• complete_parse: Keep, but without a tokenizer, things get hairy.
• comments: Keep.

[tgockel]

Change parse_error into something that ASTs can utilize. A huge amount of time is wasted in detail::parse_context::next tracking lines and columns (~15%), even though this is barely used. Beyond that, having multiple problems is more of an error with serialization than parsing (parsing can go awry quickly).