Add and apply most of rust-bitcoin's lints

[luisschwab]

Changelog

- Bump `edition` to 2021 (supported since Rust 1.56.0)
- Pins `cargo-audit` and `zizmor` versions on `Cargo.toml`, such that local and CI use the same versions
- Implement `new_hash_map` and `new_hash_set` for the `std` prelude
- Add and apply all of `rust-bitcoin`'s lints, except for documentation lints, left to a follow-up
- Add the `#![no_std]` directive such that `cargo-rbmt` will also test on a no-std target
- Bump toolchains and fix `clippy::mut_mut` lint on `Stump::serialize`

[luisschwab]

Added the #![no_std] directive so that cargo-rbmt will also test on a no-std target. This way we can drop the test-no-std CI job.

~$ cargo rbmt test

[...]

Tests complete.
Test Summary
Commit: de8c93a0b0bdd4864cb512bd5344a81a925cf2a2
  Package: rustreexo
    Examples           : stump_modify, stump_modify:std, stump_modify:with-serde, stump_modify:with-serde std, proof_update, proof_update:std, proof_update:with-serde, proof_update:with-serde std, custom_hash, custom_hash:std, custom_hash:with-serde, custom_hash:with-serde std
    Individual features: serde, std, with-serde
    Feature subsets    : [serde, std], [serde, with-serde], [std, with-serde]
+   No-std check       : ran

ref: https://git.rust-bitcoin.org/rust-bitcoin/rust-bitcoin-maintainer-tools/src/branch/master/cargo-rbmt#no_std

[luisschwab]

@Davidson-Souza latest reviews are addressed

[luisschwab]

ee6851d rebased on main

[Davidson-Souza]

Needs rebase again

[luisschwab]

e29d1fe rebased on main

[luisschwab]

eaf1f5d bumped toolchains and fixed clippy::mut_mut lint on Stump::serialize