Separated BinaryTokenizer commits from context debugging. Polished.

Separated BinaryTokenizer commits from context debugging. Polished.

Commits are relatively rare events specific to incremental parsing. Most
parsers are not incremental and do not commit/rollback. However, all
parsers need to debug what they parse. Thus, it was wrong to combine
commits with context debugging.

BinaryTokenizer single-context debugging did not support nested contexts
(such as Hello.version.major) and reported wrong FieldGroup sizes for
some parsed structures. The new BinaryTokenizerContext does not have
these problems and is more general (but still needs more polishing
work).

Also polished many field names, comments, debug messages, and some code.