Clippy warnings

[pmcgleenon]

This change addresses some clippy warnings and adds clippy to the CI

warning: manual implementation of `Option::map`
   --> src/app.rs:154:9
    |
154 | /         match state.selected() {
155 | |             Some(i) => Some(items[i].clone()),
156 | |             None => None,
157 | |         }
    | |_________^ help: try: `state.selected().map(|i| items[i].clone())`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_map
    = note: `#[warn(clippy::manual_map)]` on by default

warning: this block may be rewritten with the `?` operator
   --> src/main.rs:106:5
    |
106 | /     if res.is_err() {
107 | |         return res;
108 | |     }
    | |_____^ help: replace it with: `res.as_ref()?;`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark
    = note: `#[warn(clippy::question_mark)]` on by default

warning: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
   --> src/main.rs:142:17
    |
142 | /                 match (key.modifiers, key.code) {
143 | |                     (KeyModifiers::CONTROL, KeyCode::Char('c')) => return Ok(()),
144 | |                     _ => {}
145 | |                 }
    | |_________________^ help: try: `if let (KeyModifiers::CONTROL, KeyCode::Char('c')) = (key.modifiers, key.code) { return Ok(()) }`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_match
    = note: `#[warn(clippy::single_match)]` on by default

warning: casting to the same type is unnecessary (`f64` -> `f64`)
   --> src/main.rs:213:19
    |
213 |     let max_cpu = moving_max_cpu as f64;
    |                   ^^^^^^^^^^^^^^^^^^^^^ help: try: `moving_max_cpu`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
    = note: `#[warn(clippy::unnecessary_cast)]` on by default

warning: match expression looks like `matches!` macro
   --> src/main.rs:420:5
    |
420 | /     match nix::unistd::getuid().as_raw() {
421 | |         0 => true,
422 | |         _ => false,
423 | |     }
    | |_____^ help: try: `matches!(nix::unistd::getuid().as_raw(), 0)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_like_matches_macro
    = note: `#[warn(clippy::match_like_matches_macro)]` on by default

warning: `bpftop` (bin "bpftop") generated 5 warnings (run `cargo clippy --fix --bin "bpftop"` to apply 3 suggestions)

[jfernandez]

LGMT. Thank you for setting this up.