ambient_friendly_id

This code was adapted from friendly_id (MIT/Apache2):

https://github.com/mariuszs/friendly_id/blob/d691da682027b84eddd771d7adac0c0dc2563a35/src/base62.rs

The crate depends on failure, which is unmaintained. As we only need to generate some random IDs, we can make do by reimplementing it.