Table alteration: add untyped column

[groue]

It is already possible to create a table with untyped columns:

try db.create(table: "test") { t in
    t.column("a")
}

However, it is not possible to add an untyped column:

try db.alter(table: "test") { t in
    t.add(column: "a") // compiler error
}

This PR fixes this hole.