Optimize `Symbol::integer` by utilizing in-place formatting
This PR optimize `Symbol::integer` by utilizing `itoa` in-place formatting instead of going through a dynamically allocated `String` and the format machinery.
<details>
For some context: I was profiling `rustc --check-cfg` with callgrind and due to the way we currently setup all the targets and we end-up calling `Symbol::integer` multiple times for all the targets. Using `itoa` reduced the number of instructions.
</details>
9d272a1b05