feat: use BTreeMap for deterministic serialization

[SteveLauC]

What does this PR do

1. Replace HashMap with BTreeMap in struct Package for deterministic serialization
2. HashMap used in struct MetaCommand is also replaced with BTreeMap

   This won't affect serialization as this type is not #[derive(Serialize)]

3. Remove two extra std:: prefix in std::fmt as it is already imported

Issue

Closes #224

[oli-obk]

I think we should use this opportunity and add clippy to our CI and then deny hash maps via https://rust-lang.github.io/rust-clippy/master/index.html#disallowed_types

Would you like to tackle this as well? If not, that's ok, we can land the PR as is.

[SteveLauC]

Would you like to tackle this as well? If not, that's ok, we can land the PR as is.

Happy to include that in this PR, will do it later (tonight or tomorrow) :)

[oli-obk]

Thanks! I'll see if there are any other breaking changes that I'll want to integrate in the next release, so this may take a bit to get released as we develop the next version.