It looks like cadmium shard itself is just a proxy to other implementation cadmium shards. I.e. there is no cadmium.cr file for it to require. In your case you probably want to require "cadmium_tokenizer" as thats the sub shard that provides the tokenizer.
As pointed out in the readme, they also suggest only installing the shard you are going to use, so it might make more sense to directly install cadmium_tokenizer along with the others you want/need.
EDIT: Maybe @watzon should add a src/cadmium.cr file that requires all the other shards such that installing all of the sub shards is easier in the future.
Right, as I mentioned cadmium is an easy way to install all of the cadmium sub shards. But in practice you probably won’t need all of them, so it makes more sense to only install the sub shards you are going to use.