Implement field comparison in MongoDB

Since MongoDB 3.6 and the introduction of the new operation "expr" https://docs.mongodb.com/v4.4/reference/operator/query/expr/ it's possible to write queries with field comparison

For example

dc:format = dc:language

would betranslated into

{"$expr": {"$eq": ["$dc:format", "$edc:language]}}

We would need the support for the operators =, <>, >, <, >= and <=