fs: support utimes BigInt

### What is the problem this feature will solve? Current `utimes` FS functions have a granularity of microseconds, while the stats BigInt functionality has a granularity of nanoseconds. ### What is the feature you are proposing to solve the problem? Adding support for `BigInt` to the `utimes` functions to enable nanosecond granularity for setting timestamps. ### What alternatives have you considered? The underlying reason for this issue is that floating point second values only have 24 significant digits, which does not extend to exact nanoseconds. Having stats support `BigInt` u64 times, while having `utimes` not creates the inability to exactly set and read the same level of precision. Relying on a `BigInt` input as a u64 nanoseconds value seems the most sensible way to resolve this to me.