Inline initiative

Nobody has claimed this yet.

Add `#[inline]` attribute to relevant places

#### `#[inline]` – any obvious functions:
```rust
fn try_get_link(&self, index: T) -> Result, Error> {
self.get_link(index).ok_or(Error::NotExists(index))
}
```
#### `#[inline(always)]` – any hard delegations:
```rust
pub fn len(&self) -> usize {
self.0.len()
}
```

#### `#[cfg_attr(feature = "inline-more", inline)]` – some questionable or rarely used or internal functions:
```rust
// from united/store.rs:194
fn is_unused(&self, link: T) -> bool {
// impl . . .
}
```

Start by inspecting the Rust functions named in the issue, including try_get_link, len, and united/store.rs:194. Review nearby functions to classify obvious functions, hard delegations, and questionable or internal functions, then verify that the appropriate inline attributes and inline-more cfg_attr annotations cover the intended cases.

Written by the indexing model from the issue text.