Error with rust-nightly: type annotations needed for `Box<_>`

Error:

```
/home/xxx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/time-0.3.34/src/format_description/parse/mod.rs:83:9
let items = format_items
consider giving `items` an explicit type, where the placeholders `_` are specified
let items: Box<_> = format_items
```

I'll have a PR shortly, hopefully clippy's suggestion will fix.