Support debug-compact serializer?

[tisonkun]

We now have assert_debug_snapshot which uses "{:#?}" a.k.a. pretty print.

I wonder if it's reasonable to add a assert_compact_debug_snapshot use "{:?}" so that we may save some lines.

[tisonkun]

Currently, it's a bit wordy:

    assert_debug_snapshot!(
        candidates,
        @r###"
        [
            0,
        ]
        "###
    );

using ron reduce two spaces in the indent, but doesn't help too much.

assert_ron_snapshot!(
        candidates,
        @r###"
        [
          0,
        ]
        "###
    );

Of course, I can use assert_snapshot!(format!("{candidates:?}"), @r"[0]");, but it seems insta try to provide convenient serializer out of the box.

[max-sixty]

Waiting on review in #514 !