Quote column names

Literal expressions are now made with `init(literal:)`:

    Expression<Int>(literal: "1 + 1")

By default, `init(_:)` is for identifiers, like column names:

    Expression<Int>("id") // returns quoted identifier: \"id\"

This makes things a bit safer and more flexible.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>