allow duplicate variables in format_args_f

[ealmloff]

currently this fails to compile:

let x = 0;
rsx!{
  "{x} {x}"
}

This is because the format_args_f macro transforms it into ... format_args!("{x} {x}", x = x x = x) ... which contains duplicates that are not allowed in format_args. This PR deduplicates the assignments.

[mrxiaozhuox]

cool!

[jkelleyrtp]

Thanks!