(mixal-mode-syntax-table): Add \n as end-comment.

(mixal-mode-syntax-table): Add \n as end-comment.
(mixal-operation-codes): Remove.
(mixal-operation-codes-alist): Immediately initialize to full value.
(mixal-add-operation-code): Remove.
(mixal-describe-operation-code): Make the arg non-optional.
Use the interactive spec instead.
Use mixal-operation-codes-alist rather than mixal-operation-codes.
(mixal-font-lock-keywords): Don't highlight comments here any more.
(mixal-font-lock-syntactic-keywords): New var.
(mixal-mode): Use it.  Fix comment-start-skip.