Remove the need of writing to_string() everywhere in the tests by using `big_s`

[bidoubiwa]

Currently, throughout the tests we are adding to_string() everywhere. Which is tedious and reduces readability.

To clean the code, we should use the crate big_s

Todo

• Add big_s to the repo as a dev dependencies cargo add big_s --dev

• Import the library in every tests mod that needs it:

  mod tests {
   use big_s::S;
   //...

• Change all occurrences of for example "something".to_string() to `S("something")

[romilpunetha]

Why is this being used only for tests?

[bidoubiwa]

@irevoire may answer that