Add rust code for backtracking problems

[nikhilmitrax]

Mainly porting the python algorithms, and trying to follow similar paradigm as the python code.

Mostly written by hand, albeit with AI autocomplete on.

[Destiny-02]

Hi @nikhilmitrax, thank you for writing the Rust solutions for the Backtracking chapter! Looks good overall, just a few comments for you to check.

[Destiny-02]

The existing rust solutions in the Sliding Windows chapter don't use pub fn. If there is a reason to use pub fn instead of fn, please update the solutions in Sliding Windows. Otherwise, we can stick to fn.

[nikhilmitrax]

I believe pub fn makes more sense since the functions aren't really useful without having public visibility and being isolated in a file.

As is, the rust functions aren't particularly usable without a module setup and at least a project definition (Cargo.toml)

I see 3 potential paths moving forward, and I'm perfectly fine with all 3 options, do you have a preference?

Options:

1. Remove pub everywhere
2. Add pub to all functions that should be usable outside a file.
3. Add pub to all functions that should be usable outside a file, and add a Cargo.toml (possibly along with lib.rs and some tests, so that cargo test runs through the examples)

[Destiny-02]

Option 1 please!
Here is some background if you're interested: #54 (comment)