Rename first argument Option::unwrap_or to default

We [currently](https://doc.rust-lang.org/std/option/enum.Option.html#method.unwrap_or) use the definition for `Option::unwrap_or`: 

```rust
pub fn unwrap_or(self, def: T) -> T
```

I sometimes read `def` as `define`. It would be clearer for me (and hopefully others!) if it had the following signature: 

```rust
pub fn unwrap_or(self, default: T) -> T
```