Clippy: implement suggested code improvements

[duskoo]

cargo clippy -h
Checks a package to catch common mistakes and improve your Rust code.

Example clippy suggestions for app-backend-rust repo:

warning: this let-binding has unit value
  --> src/ios/ios_interface.rs:43:5
   |
43 |     let _ = simple_logger::setup_logger(filter_level, coepi_only);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: omit the `let` binding: `simple_logger::setup_logger(filter_level, coepi_only);`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value

warning: using `clone` on a `Copy` type
  --> src/reporting/public_report.rs:46:21
   |
46 |         let other = inputs.ids.contains(&SymptomId::Other).clone();
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try removing the `clone` call: `inputs.ids.contains(&SymptomId::Other)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy

Tasks:

• run cargo clippy and apply suggested code improvements.
• add clippy as continuous integration step in Github (see Index boundaries TCNCoalition/TCN#82 for reference)

[ivnsch]

Let's create a separate issue for the CI (in general). Ideally we should use Github actions like the TCN repo.

Edit: CI issue: #90