Add FromHex implementation for types that implement ToHex
Some structs, such as `PaymentId`, implement the `ToHex` trait from the `hex` crate. For example, see https://docs.rs/monero/latest/monero/util/address/struct.PaymentId.html#impl-ToHex-for-PaymentId

For those types that implement ToHex, it also makes sense to implement `FromHex`, from the `hex` crate (https://docs.rs/hex/latest/hex/trait.FromHex.html).