Add compile-checks to ensure the parameter to absl::numbers_internal::FastIntToBuffer isn't provably insufficient

This performance-sensitive API is internal, and should not be used directly. Nevertheless, some do use it, so it's worth trying to block incorrect uses where we can.

PiperOrigin-RevId: 594289730
Change-Id: Ib5df39b450ef2ceda795082cde10ec2ba2642172