Stop using Symbols

[jwoertink]

Related: #1061

While working on #1059 I'm having to do a lot of transforming since columns will need to be wrapped in quotes. Since we're using Symbol all over the place, I have to make sure I convert in to String, and then it ends up getting used in a string anyway... I thought we had an issue already talking about this, but I'm not sure where it's at.

On the one hand, this looks nice add_index :users, [:email], but it's not much more than add_index "users", ["email"]. The other issue is with a Symbol is we lose the ability to do checks like .ends_with? without first converting to a String anyway.

I know in some cases we allow both Symbol and String, but why not just enforce a single type for consistency and reduce a few Unions in the code.