Allow duplicates in bson documents

[AlekSi]

What should be done?

bson package should support documents with duplicate keys, even other layers reject them. That should make testing and fuzzing easier.

Where?

In bson package (just like in types package). Handlers are not in the scope.

---

• Find the current issue's TODOs in the source code and modify them.
• Check skipped tests.
• Diff tests are written for insert.

Implementation comments

• For insert: if there are duplicate keys, an error is returned (this should be documented in diff.md)
• For update operators: if there are duplicate keys in the update document, an error is returned (see examples in TestUpdateFieldCompat* where * is an operator).