Either support or error when `items_capacity=1`

[dcherian]

I have a silly workload where I really just wanted a cache size of 1. However this doesn't work since the shards end up having a capacity of 0. It turns out the smallest number that can work is 2. I think the library should either round up 1 to 2 or raise an error to prevent this footgun.

[arthurprs]

Fixed in #106 I'll produce a new minor release with it shortly.

[dcherian]

thank you!