`std::error::Error` implementation for `hex::Error`

[Velnbur]

It seems that elements::hex::Error is missing std::error::Error implementation, which makes it difficult to work with dynamic-error crates (like eyre or anyhow), as there default ::from method requires that.

For example,

let bytes_res = Vec::<u8>::from_hex(&raw_tx).map_err(eyre::Report::from);

doesn't compile, as eyre::Report::from requires T: std::error::Error, but:

let bytes_res = Vec::<u8>::from_hex(&raw_tx).map_err(|e| eyre::eyre!(e));

works, as it does some black-magic under the hood with the value. So still from implementation is preferable

[apoelstra]

We should really just drop this module entirely in favor of hex-conservative.