Add direct support for i128 and u128 via itoa

[Lucretiel]

itoa natively and unconditionally supports i128 and u128, so we can use it instead of to_string on those types.

This change should have no publicly visible behavior changes. It was previously impossible to customize behavior for 128 bit ints in Formatter, so there aren't any public facing behavior changes introduced by the addition of new methods to Formatter.

The MSRV for serde_json is 1.36, so there should be no need to conditionally compile anything in this MR. This is affirmed by the itoa dependency, which unconditionally supports 128 bit integers.

[dtolnay]

Thanks!