once_cell 0.1.8

Single assignment cells and lazy static values without macros.

# Changelog

## 0.1.8

- update crossbeam-utils to 0.6
- enable bors-ng

## 0.1.7

- cells implement `PartialEq` and `From`
- MSRV is down to 1.24.1
- update `parking_lot` to `0.7.1`

## 0.1.6

- `unsync::OnceCell<T>` is `Clone` if `T` is `Clone`.

## 0.1.5

- No changelog until this point :(