`uniq` of a scalar should be an array of the scalar

[epage]

Example test:

    assert_eq!(v!(["foo"]), filters!(uniq, v!("foo")));

• Rust implementation
• Ruby implementation
• Relevant tests
  • Once this is working, ideally the test will start failing. We would just need to remove the #[should_panic] on it
  • If the test doesn't start failing, please temporarily remove #[should_panic], and find or create an issue for the new failure

[epage]

The macro introduced here (as_sequence) should help fixing #246, #255, #256, #266 (and maybe others). It helps writing filters that take a value and "coerce" it into an array the same way Ruby's InputIterator does.