Enhance SQLLiteral and SQL interpolation again

[groue]

This pull request improves over #690, so that it gets easier to define functions that build literal expressions with SQL interpolation:

func date(_ value: SQLExpressible) -> SQLExpression {
    SQLLiteral("DATE(\(value))").sqlExpression
}

The inspiration for this change comes from #705 by @martindufort. Support for the INSTR SQLite function can now be defined as below:

func instr(_ lhs: SQLExpressible, _ rhs: SQLExpressible) -> SQLExpression {
    SQLLiteral("INSTR(\(lhs), \(rhs))").sqlExpression
}

// Usage: SELECT * FROM "player" WHERE (INSTR("name", 'foo')) > 0
let players = try Player
    .filter(instr(Column("name"), "foo") > 0)
    .fetchAll(db)