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.
Is there an issue with your quote matching?

~~~rust
asm!("syscall" : "{rax}" = n -> ret, {rdi}" = a1, "{rsi}" = a2, "rcx", "r11", "memory", "volatile");
~~~

vs.

~~~rust
asm!("syscall" : "{rax}" = n -> ret, "{rdi}" = a1, "{rsi}" = a2, "rcx", "r11", "memory", "volatile");
~~~

Also, why is there a `:` in there? Shouldn't that be a comma? Or am I misunderstanding?