Merge pull request #158 from tayloraswift/collection-agnostic-aggregate

enable `$aggregate` to run without a collection

Merge pull request #157 from tayloraswift/upgrade-nio-78

upgrade to SwiftNIO 2.78

Merge pull request #156 from tayloraswift/upgrade-bson

upgrade swift-bson to 0.9.0

downgrade SwiftNIO to 2.77.0 to unfail macOS build

upgrade swift-bson

Merge pull request #155 from tayloraswift/upgrade-swift-bson-0-5

upgrade swift-bson (again)

Merge pull request #154 from tayloraswift/upgrade-swift-bson

upgrade swift-bson dependency

upgrade swift-bson

Merge pull request #152 from tayloraswift/spinoff-packages

spinoff packages and add dependency on swift-bson

Merge pull request #151 from tayloraswift/predicate-configurable

factor out `Mongo.PredicateEncoder` APIs into a protocol