Fixes compatibility with the `digest` crate.

Note: `hmac` already received a v0.13.0-rc.6 release in #256 which
includes fixes for the same issue.

Releases the following:
- `cbc-mac` v0.2.0-rc.5
- `cmac` v0.8.0-rc.5
- `pmac` v0.8.0-rc.5

Closes #257