Previously we stored an `Arc<Box<dyn Fn()>>`, but we can directly store
the function in the `Arc`.

We could also take an `impl Fn() -> f64 + Send + Sync` instead of a
`Box<dyn Fn() -> f64 + Send + Sync>` and do the allocation inside `new`:

```rust
impl PullingGauge {
    pub fn new<S1: Into<String>, S2: Into<String>>(
        name: S1,
        help: S2,
        value: impl Fn() -> f64 + Send + Sync,
    ) -> crate::Result<Self> {
        Ok(PullingGauge {
            value: Arc::new(value),
            desc: crate::core::Desc::new(name.into(), help.into(), Vec::new(), HashMap::new())?,
        })
    }
}
```

This is more efficient and lets you write the closure directly on the
calling site:

```rust
// Previously
let pulling_gauge = PullingGauge::new("foo", "bar", Box::new(|| static_vec.len() as f64));
// Now
let pulling_gauge = PullingGauge::new("foo", "bar", || static_vec.len() as f64);
```

This wouldn't be a breaking change since `Box<dyn Fn()>` implements `Fn()`,
but it would make it less efficient since you are now storing a `Box`
inside the `Arc`.

Signed-off-by: Jalil David Salamé Messina <[email protected]>