Heads-up: const_err lint is going away

[RalfJung]

This crate has a deny(const_err). That is a no-op in current compilers, const_err is deny-by-default (and has been for years). In the near future, the lint will be turned into a hard error, at which point this crate will fail to build since the deny(warnings) means that the warning due to the removed lint becomes an error.

The deny(const_err) should probably just be removed.