feat: no-allocation hex-encoding using `HexWriter`

[elsirion]

HexWriter allows to directly hex-encode binary data to a std::fmt::Write object from any source that can write to a std::io::Write object. This saves between one and two allocations depending on the use case.

Let's assume a function Foo::encode(&self, writer: std::io::Write) -> Result<(), std::io::Error>:

• Writing a Foo object to Formatter as hex would require 2 allocations: one to encode bytes into a Vec<u8> and a second to encode the bytes into a String before writing it to the Formatter. Here we can save 2 allocations.
• Writing a Foo struct as hex into a String, here we only save 1 allocation (the intermediate byte step)
• Writing a Vec<u8> as hex to a Formatter: 1 allocation saved (String)

[elsirion]

@dpc this project might be unmaintained, should we just publish a fork?

[dpc]

@elsirion Rust Bitcoin has a separate hex crate now or something. Maybe we can just add it there. Maintaining creates is a lot of ungrateful work. 🤣