[WIP] Log macro kv

[Thomasdezeeuw]

This is my attempt at modifying the log macro to add support for key values, as described in #343 (comment). I didn't get very far as I'm not the best at Rust macros.

Testing with cargo test --features std kv_unstable gives the following error:

error: local ambiguity: multiple parsing options: built-in NTs tt ('arg') or 1 other option.
  --> tests/kv_macro.rs:23:87
   |
23 |     log!(target: __log_module_path!(), Level::Info, "some {} message {}", "great", 1, {
   |                                                                                       ^

For #343.

/cc @KodrAus @yoshuawuyts

[Thomasdezeeuw]

Closing in favour of #353.