Remove identity casts from optimizer

[zachyee]

https://github.com/prestodb/presto/issues/4259

@maciejgrzybek

[maciejgrzybek]

from optimizer is not necessary in commit msg. What about Add remove identity casts rewriter? Commit msgs guidelines: http://chris.beams.io/posts/git-commit/

[maciejgrzybek]

Back to you, @zachyee.

[zachyee]

I addressed the easy-to-fix comments. I responded with some info on the harder-to-fix comments to figure out what would be best before trying to address them. @maciejgrzybek