Implementation of #70, working in progress.

Steps

• Add kv::Key and kv::Value
  • Let's try using value-bag directly (alias) for value first.
• Add kvs field to struct Record.
• Logging macros accepts kv optional parameter.
  • I plan to use this syntax

    info!(kv: { a = 1, b = 2 }, "payload {}", 123);
    info!("payload {}", 123, kv: { a = 1, b = 2 });

    The kv is optional, just like the existing logger argument, and I may wish to allow them to be in any order. The colon allows us to distinguish with the formatting arguments so that it can be placed anywhere.
  • We can consider whether the following more advanced usages will be supported in this PR or in the future, but these syntaxes have not yet been decided.

    let key = "a";
    info!(kv: { {key} = 1, b = 2 }, "payload {}", 123);
    
    let kvs = HashMap::from_iter([("a", 1), ("b", 2)]);
    info!(kv: kvs, "payload {}", 123);

• Add tests.
• Support it in FullFormatter.
• Support it in PatternFormatter.
  • How to design placeholders for KV? - Simply adding a new {kv} pattern to output like "{ k1=1, k2=text }"
• Serialize KVs to a separated kv field in JsonFormatter.
• Key modifier in macros.

  info!(kv: { k1: = value, k2:? = value }, "message"); // "message { k1=display, k2=debug }"

• Shorthand in macros.

  let num = 123;
  info!(kv: { num }, "message"); // "message { num=123 }"

• Support parameter arbitrary ordering. (might require to refactor it in proc-macro)
• Try to replace slice with phf map for KV. (self-reminder: benchmarking it..)
  • phf only supports limited types for key (doc), and we have a custom type kv::Key, so abandon the idea.
• Add documentation and examples (macros, accessing in Record, downcasting, etc.).