rewrite in rust

rewrite in rust

pmg-log-tracker has been rewritten in Rust. Functionality is the same.
Output sometimes has a different order than the pmg-log-tracker in C.
This only happens when the time of the entries match.

There's one change regarding the interface. In addition to the short
versions of arguments also long versions exist.

The implementation uses Rc<>, Weak<> and RefCell<> to make holding mutable
cross-references possible, without having to change the original logic
completely. This allowed for easier translation from C to Rust.

The file debian/cargo-checksum.json is required by dh-cargo, otherwise
it won't compile. The cargo-checksum.json should contain the upstream
.crate file which does not exist in this case, so we just create an
empty one with the required keys. (see 'Library package structure' in
https://wiki.debian.org/Teams/RustPackaging/Policy)

The change to the minimum version of debhelper required was done
according to other rust packages (rust-clap, rust-bindgen, rust-ripgrep).

Adds a README that provides an overview of the stages a mail passes
through and what we can use to match those together for a single mail.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>