Use custom_debug_derive instead of the Hex/HexSlice wrappers

[jonas-schievink]

Hex wraps a value that implements fmt::LowerHex, and uses that implementation when fmt::Debug-printed ({:?}). HexSlice takes any type implementing AsRef<[u8]> and prints it as hexadecimal when printed via fmt::Debug/{:?}.

These types make it easy to quickly customize how fields are printed by wrapping them in Hex/HexSlice, which is way less work than implementing fmt::Debug by hand (and less error-prone, because #[derive(Debug)] will never forget any fields, unlike a hand-rolled impl).

The problem with them is that they conflate data with behavior by being embedded in the struct/enum definition itself instead of being part of an impl or being attributes. It takes an extra step to access a value behind them, too.

I think it would be a good idea to use custom_debug_derive instead. For that, we should add a hex and hex_slice function to utils that do what Hex and HexSlice currently do, and then use #[debug(with = "utils::hex")] on the fields that are now wrapped in Hex and #[debug(with = "utils::hex_slice")] on fields that use HexSlice.

That crate also supports specifying a format string, which is going to be quite a bit less efficient than what we have now, so it should only be used when necessary.