`compact` filter doesn't properly work on hashes

[epage]

Example test:

    let assigns = v!({
        "words": ["a", nil, "b", nil, "c"],
        "hashes": [{ "a": "A" }, { "a": nil }, { "a": "C" }],
    });

    // Test hashes
    assert_template_result!(
        "A C",
        "{{hashes | compact: 'a' | map: 'a' | join}}",
        assigns
    );

• 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]

This solves issues #335. It also solves some of issue #246, but the filter doesn't yet work on a single hash.