RFC: refine the `asm!` extension

[Rendered view](https://github.com/pczarn/rfcs/blob/master/active/0000-asm-format-macro.md)

An existing implementation: https://github.com/pczarn/code/blob/master/rust/asm/lib.rs

Some polishing of the implementation is necessary. It doesn't handle corner cases and incorrect input that well.