Implement `From<Partial::Item>` for `Partial + Default` using `apply_some`

It should be possible to rework things slightly so that we can go from a `Partial::Item` into the base struct, when the base struct implements `Default`.

Example

```
use partially::Partial;

#[derive(Default)]
struct Base {
value: String,
}

#[derive(Default)]
struct PartialBase {
value: Option,
}

impl partially::Partial for Base {
type Item = PartialBase;

#[allow(clippy::useless_conversion)]
fn apply_some(&mut self, partial: Self::Item) {
if let Some(value) = partial.value {
self.value = value.into();
}
}
}

impl From for Base {
fn from(value: PartialBase) -> Self {
let mut obj = Base::default();

obj.apply_some(value);

obj
}
}

#[test]
fn into_test() {
let full_partial = PartialBase {
value: Some("modified".to_string()),
};

let base: Base = full_partial.into();
}

```

Start with the Partial trait and its apply_some method, then compare the requested conversion with the example in the issue. Done means the shown PartialBase value can be converted into a Base with Base::default and apply_some, and the example test passes.

Written by the indexing model from the issue text.