Signed-off-by: gcarq <egger.m@protonmail.com>

Signed-off-by: gcarq <egger.m@protonmail.com>

Add Rust syntax highlighting to `README.md`

This gets rid of two security advisories [1, 2]
reported by `cargo audit` [3]. After this commit,
no other vulnerabilities are reported by the tool.

This is a heavily breaking change, as the `Callback`
trait had to be adapted.

We prefer to keep the diff as minimal as possible,
rather than strive for the maximally idiomatic code.
Thus, we refrain for now from optimizations such as
deriving `clap::ValueEnum` on
`blockchain::parser::types::CoinType`, or using
`clap::command!()` to read the crate author and
version from the Cargo manifest.

We reshuffle the argument parsing code a bit in
order to be able to add a set of unit tests that
assert some basic desired behaviors.

The updated help text is reflected in the diff of
the README file.

[1] https://rustsec.org/advisories/RUSTSEC-2021-0139

[2] https://rustsec.org/advisories/RUSTSEC-2021-0145

[3] https://crates.io/crates/cargo-audit

bump clap dependency to 4.x

Signed-off-by: gcarq <egger.m@protonmail.com>