use internal _bson_get_len from swift C bson bindings

This method is introduced to overcome limitations around field
access when `bson_t` is treated as an `OpaquePointer`.

Author: mbroadst

Package.resolved

@@ -6,8 +6,8 @@ let package = Package(
         .library(name: "MongoSwift", targets: ["MongoSwift"])
     ],
     dependencies: [
-        .package(url: "https://github.com/mongodb/swift-bson", from: "2.0.0"),
-        .package(url: "https://github.com/mongodb/swift-mongoc", from: "2.0.0"),
+        .package(url: "https://github.com/mongodb/swift-bson", .upToNextMajor(from: "2.0.0")),
+        .package(url: "https://github.com/mongodb/swift-mongoc", .upToNextMajor(from: "2.0.0")),
         .package(url: "https://github.com/Quick/Nimble.git", from: "7.3.0")
     ],
     targets: [

Package.swift

@@ -284,7 +284,11 @@ extension Document {
     public var rawBSON: Data {
         // swiftlint:disable:next force_unwrapping - documented as always returning a value.
         let data = bson_get_data(self.data)!
+#if compiler(>=5.0)
+        let length = _bson_get_len(self.data)
+#else
         let length = self.data.pointee.len
+#endif
         return Data(bytes: data, count: Int(length))
     }