Optimise TableSyntax (dart-lang/markdown#472)

* Optimise TableSyntax

* Update a comment

* Update tables.unit

* Set growable false for columns

* Simplify neverMach

* Update gfm_stats.txt

* Optimise TableSyntax

* Update a comment

* Set growable false for columns

* Simplify neverMach

* Optimise performance

* More optimisation

* Fix some requests

* Add a comment

* Rename align to alignment

* Fix a request