fmt: Refactor the arg fetching code
fmt: Refactor the arg fetching code

* Error out if the requested index is out-of-bound
* Tidy-up all the arg-related variables in a struct

raulgrell/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig