Support $ (positional) array update operator

[AlekSi]

Depends on #503 (potential conflicting changes between two array operators).

https://www.mongodb.com/docs/manual/reference/operator/update/positional/

DoD:

• update (ideally, updateOne and updateMany test cases) and findAndModify
• Cases for unsets, negations, and multiple arrays are covered
• Test case with duplicate values in the array (only first one needs to be modified)
• Test case when empty field is set and field doesn't exits
• Addressing fields through foo.$.bar is covered
• Update user docs