Provide convenience initializer via `ExpressibleByStringLiteral`

[jberkel]

public protocol ExpressionType : Expressible, ExpressibleByStringLiteral {
}

let date:Expression<Date> = "date"

[jberkel]

This interacts badly with the current operator handling (and offers little value):

Expression<String>("foo") + "bar" 
// compiler treats this as
Expression<String>("foo") + Expression<String>("bar")
// "foo" || "bar"  (actual)
// "foo" || 'bar'  (expected)

A better option might be to use property wrappers, see #976