Use iter instead of into_iter
Use iter instead of into_iter

Clippy emits:

  warning: this `.into_iter()` call is equivalent to `.iter()` and will
  not consume the `slice`

As suggested; call `iter` instead of `into_iter`. Requires some slightly
funky syntax to avoid taking ownership in the error path.

apoelstra/rust-jsonrpc,apoelstra/rust-jsonrpc