Don't require entity ids to be integers

Do we need entity ids to be integers? Consider improving flexibility to use char ids.

This was previously initiated in #585, we have now started discussing how to do this.

Start by reviewing the discussion referenced in issue #585 and the current discussion in this issue. Identify the affected code paths and tests before implementation; done means character entity IDs work wherever entity IDs are accepted without breaking integer IDs.

Written by the indexing model from the issue text.