Proc-macro: Rationalize function signature logic (transform_sig)

[taqtiqa-mark]

Once proc-macro-attribute #[trace(...)] has test coverage and documentation covering the relevant code, it may be possible to simplify the logic within minitrace-macro/lib: transform_sig(...). Specifically, by eliminating these lines, (per this observation in PR #119):

minitrace-rust/minitrace-macro/src/lib.rs

Lines 259 to 275 in 819d611

	for (i, arg) in sig.inputs.iter_mut().enumerate() {
	match arg {
	FnArg::Receiver(Receiver {
	reference: Some(_), ..
	}) => {}
	FnArg::Receiver(arg) => arg.mutability = None,
	FnArg::Typed(arg) => {
	if let Pat::Ident(ident) = &mut *arg.pat {
	ident.by_ref = None;
	} else {
	let positional = positional_arg(i, &arg.pat);
	let m = mut_pat(&mut arg.pat);
	arg.pat = parse_quote!(#m #positional);
	}
	}
	}
	}

[taqtiqa-mark]

Voided by PR #127 rejection.