pub const KEYS_MAX: usize = _; // 65_536usizeExpand description
How many names one collection can intern.
An id is two bytes because that is what an interned object’s key entry is,
so the table stops at 65536 names. A collection that reaches it is not a
collection of documents any more, it is a collection of a schema per row,
and Keys::intern answers None so the writer can store that document
with its keys as bytes rather than fail the write.