Looks up a code's character mapping (cidchar, cidrange), in this CMap and then its parents.
public bool TryGetCid(CharacterCode code, scoped out int cid)
A code read by ReadCode(ReadOnlySpan<Byte>).
The CID; 0 when there is none.
ISO 32000-2 §9.7.6.2; Adobe TN 5014 §5.4 (a using CMap's mappings win over its parent's).